Zeus vs. Daghdha

Not while the Celtic gods were being worshiped there, though, except on worlds like Gary Gygax's Epic of Ærth campaign, where the British Isles never stopped worshiping the gods of the Celts. And of course here we get into the iffy (for these forums) question of whether Christianity exists anywhere in the Planescape multiverse. If it does, I think the Celts and the Norse would see it as a common foe, rather than one another. Ærth (a world Gygax invented after he left TSR) is a sort of parallel Earth where fabled lands like Atlantis, Ys, Lemuria, and Lyonesse never sank beneath the ocean, and Christianity never took hold. It advanced to something like the early Renaissance technologically and stopped there, but it has magic and many portals to the world of Faerie. If that world had an equivalent of the Hundred Years War between France and England, the Roman and Celtic pantheons would have again been in conflict, as Ærth's equivalent of England somehow managed to continue worshiping the Celtic faith.

There aren't many examples in D&D of prime worlds where the historical Earth pantheons are worshiped. The "Historical Reference" series for 2nd edition assumed a more or less historical world, so Christianity did exist, or would eventually come to exist. It's usually assumed each pantheon gets a world all to itself, which would minimize interpantheonic conflicts. Apart from that, all I can think of are the smattering of Earthly deities revered on Toril, and the world of Thesalys in the sphere of Greatspace, where the Greek pantheon is worshiped. More worlds like Ærth are needed to help justify conflicts between pantheons.

Oh, he probably is. It's good to be king. I don't think that's the same as being a trickster, though. Note that On Hallowed Ground gives him a pure neutral alignment, rather than any sort of chaotic.

The Sumerian and Babylonian pantheons don't get along, especially since Nergal and Anshar slew Enki. I suspect the Babylonian and Egyptian pantheons are both still mad at the orc gods, who slew many of their avatars during the Orcgate Wars of Toril. I don't think the Faerunian pantheon is unified enough to share any group blame for the conflicts between their gods and the Egyptian gods of Mulhorand.

The Egyptian and Greek pantheons are an interesting possibility. I'm not aware of anything about it in canon, and historically I don't think they actually conflicted that much; there was a lot of syncretism between them, and Isis became really popular in Rome. But there was syncretism between the Roman and Celtic gods, too.

The Finnish god Ukko is said to be rapidly losing followers to the Slavic gods Dazhbog and Perun in Eastern Europe (Dragon #290).

Set married two members of the Canaanite pantheon, according to Wikipedia, and that might have caused all sorts of political trouble.

Following the historical logic, then the Celtic gods should be at odds with the Norse gods too since the Vikings colonized a lot of the same territory at a later point in time.
Don't forget what the Saxons did to the Celts.

but I always pictured Zeus as being laughing and boisterous
Sometimes he is (e.g. the case where he stupidly threw off his mortal disguise and was all "It's me, Zeus!" before his mistress, whereupon she was struck by lightning emanating from his body) and sometimes he's a complete ass (see the whole fit over Prometheus giving humans intelligence and fire, and Zeus punishing the humans by sending them the first woman, Pandora *you have to read that part of the Hesiod to understand why giving men the first woman was an asstastic thing to do-- yeah, that work depicts women as being quite evil.*. Besides that, the humans didn't do anything wrong; Zeus basically unloaded some wrath upon the humans for what Prometheus did.)
As bad as Zeus is however, Hera is far worse. She's essentially the Greco-Roman pantheon equivalent of caricaturized Hillary Clinton, I kid you not. (for those too lazy to read, she has a very "evil stepmother" thing going on with Artemis, she refused to have sex with Zeus for a full year out of rage when Athena was born, and she's constantly torturing and killing Zeus's mistresses. In one instance, she even friggin' gets revenge on the mistress by killing her children instead of tormenting her!)
Also, let's not forget that not all of the ladies that Zeus and Dionysus chased after were receptive to their advances.
BTW, Aphrodite is pretty screwed up, too (read the story about Adonis for details; creepy as hell. It also puts an amusing spin on the disney cartoon series about Hercules, LOL.)

Are there any other "non-obvious" conflicts amongst the pantheons I should know about? E.g. Egyptian vs. Forgotten Realms, Isis vs. Hera (man, those Romans were everywhere), etc.
Don't forget Forgotten Realms vs. Maztica.

Are you talking about Semele? That's the only occasion I can think of where Zeus killed a consort by revealing his true form. But in the version of the myth I read, Semele agreed to swallow the heart of Zagreus, Zeus's titan-slain son, in order to bear him to term, and he took her as a consort. But Hera was especially angered about this consort relationship because Zagreus was originally conceived from Zeus sleeping with Persephone. In the heat of passion, Zeus had sworn to give Semele anything she wished. And Hera in disguise had told Semele he was Zeus and convinced her to ask Zeus to reveal himself in all his glory and prove it. And since a god can't break their oath, he did so, and Semele was killed as you described. (Then Zeus took the heart back from the remains, stitched it into his thigh, and eventually his child was reborn as Dionysus.) It wasn't Zeus playing some kind of prank on Semele at all, and he didn't even want to do it, he was forced into it.

(Honestly, the more I read about Greek mythology, the more I really question the pantheon being described as CG)

Not just them. Even Hephaestus tried to force himself on Athena and ended up accidentally conceiving Erichthonius with Gaia in the process.

Also, if anyone's interested, I've had the Norse and Eygptian basically headhunting Kyuss in my own campaigns.
I doubt beheading Kyuss would do them any good. He'd just regrow a body of undead, prehensile, interwoven, ambulatory worms. That said, I think Anubis and Kelemvor would be particularly opposed to Kyuss.

Are you talking about Semele? That's the only occasion I can think of where Zeus killed a consort by revealing his true form. But in the version of the myth I read, Semele agreed to swallow the heart of Zagreus, Zeus's titan-slain son, in order to bear him to term, and he took her as a consort.
Never heard that before. In the version I read, the consort died, but had conceived, and Zeus saved the zygote by attaching it to his leg and bringing it to term. Can't remember if it was Semele or not (probably was, though). And yeah, Hera wanted that boy dead, at any rate.

And Hera in disguise had told Semele he was Zeus and convinced her to ask Zeus to reveal himself in all his glory and prove it. And since a god can't break their oath, he did so, and Semele was killed as you described.
Eh, the version I read (one of the versions on Theoi.com, anyway), Zeus basically revealed himself (just couldn't resist doing so) in ignorance and she was struck by lightning, and he felt really horrible about her death. In that version however, he actually attached the zygote (it sure as hell wasn't a fetus) to his leg and brought it to term. In his defense, this was supposed to be one of his first dalliances with a mortal female. In at least one of the sources, it sounded as though Zeus's true form WAS lightning, so I'm thinking that particular version was much older than most. As for his reason behind the disguise in that version, my understanding was that it was to avoid the pitfalls of celebrity in order to mingle with the mortals a bit.

Not just them. Even Hephaestus tried to force himself on Athena and ended up accidentally conceiving Erichthonius with Gaia in the process.
Yeah, but it's not QUITE the same thing since the marriage was arranged by Zeus, and back then it wasn't considered rape if you were married to the woman.

The Norse pantheon has its own alignment problems. I mean, look at what Thor did to Loki over the death of Baldur. While most versions you read are somewhat sanitized and just say "ropes" or "chains", in the older/original versions...
Thor punished Loki by turning one of his sons into a wolf. That son killed the other son. Then, Thor ripped out the intestines of the (now slain) non-polymorphed son, and placed a spell on them to make them as tough as iron. He used the intestines to bind Loki to the rock under the Midgard serpent's mouth until Ragnarok. Talk about evil. Hell, that's not just evil, that's VILE.
Then of course there's the time that Loki was kidnapped by that one frost giant. Thor got revenge not only by killing said frost giant, but he also slew his daughters, and every Frost Giant man, woman, or youth he encountered. Who of course had absolutely nothing to do with Loki's kidnapping.

Plus prettymuch every single religion from the BC's era has the gods of sun/sky/righteousness punishing evildoers by subjecting their offspring-- even babies-- to painful deaths.

As far as the Olympians however, I would say that Hera by far is the 2nd most evil of them (behind only friggin' Ares)

Oh, not a zygote or fetus in the one I read; literally Zagreus's heart still.

Are you sure about that? I've never heard of Hephaestus being married to Athena, only Aphrodite, with her cheating on him with Ares on the side. Remember the whole thing about Hephaestus trapping Aphrodite and Ares in his own bed to humiliate both of them during one of their dalliances?

I dunno, Athena could be pretty cold. The Arachne incident alone is a good example. Yes, Arachne was being all filled with hubris in saying she was better than Athena, but she was right, even Athena grudgingly admitted it. And she wasn't claiming it to shame Athena; when she did that final weave that got her punished, she wanted Athena to be proud of her, since Athena'd always claimed that to improve your skills was the ultimate good. But nope, she couldn't have that, so SPIDER'D.

But even worse than that, look at Troy; it was the city that held her Palladium, the city she had promised to always protect so long as it was held within its walls. So what does she do? She gets so spiteful towards the Trojans after the whole "who's the fairest" incident - something that really shouldn't even matter to her considering she'd never really cared about personal beauty anyway, she saw it as too feminine a concern to worry about; she just couldn't stand the insult - that she tells some of its heroes "oh, you don't need to worry, you are under my protection remember :)", appears to the Greeks and powers them up in order to let them slaughter Trojans better, and even helps Odysseus and Diomedes to sneak into Troy and steal the Palladium just so that she isn't breaking her own word. Then she manages to talk Zeus into getting so fed up over the whole thing that he essentially withdraws his own support of Troy, just to make sure as many Trojans as possible can be slaughtered.

When Aphrodite appeared during the war to try and protect Aeneas, her son, from being killed in battle by Diomedes, Athena powered Diomedes up enough to stab her and actually wound her, to get her to abandon him and try to escape. Then when Zeus pulled her back to Olympus, she and Hera mocked her to tears by claiming she couldn't have been wounded in battle, she must have been sneaking around the Trojans looking for some men to lay with, and pricked herself on a sewing pin.

Hera might be conniving, and Ares might be wrathful and blood-crazy, but Athena is petty and calculating.

Still, even though they've all done some pretty awful things (except maybe Hermes?), I honestly wouldn't call all of them evil. I'd say neutral, in the sense that they can act in either way. Old goofy 2e-style CN, I suppose, as annoying as it was. Ares is awful, and Hera I can't really think of any occasions where she acted well (though I can think of many of the things she did as being entirely defensible, if you consider just how awful Zeus was to her as a husband, and the perspective the gods had as a whole that mortals didn't really matter that much), but most of them could commit merciful acts as easily as wrathful ones.

You know, I've thought on that too, and I think there really is a lot of potential there. Remember that it's not just the Celts and Vanir either (but don't forget that the Aesir and the Vanir both hate the giants); the Seldarine kicked the giant deities out of Arborea way back when and took their realms for their own, before Lolth had left the pantheon even. And with how many titanspawn are giant-kin, and with the similarities between giants and titans as it is, I can easily imagine the Greeks not caring much for them too.

There's a lot of stuff you could do with that; it really is pretty unfair to such a (for the most part) peaceful pantheon that pretty much everyone in the CG region of the ring seems to dislike or even hate them to the point of stealing their ancestral land, and it'd be a really interesting thing to do with a pantheon that's never had as much influence in story stuff as its size should have given it. It'd be one of the few times you could paint the "good guys" as villains legitimately, and honestly that's always an interesting way to go to me.