I'd prefer to see Urban Planescape approached with a more serious tone in it's writing than simply silly. While I don't mind the aspects of the surreal that Planescape at times achieves, I don't want to see an idea like this slide into Saturday Morning Cartoon level. So whiel a few silly/goofy ideas here and there are fine - in the long run, I'd like to see serious ideas dominate.
Others opinions?
Linked to the "Thread of Threads", because I feel that this is something important for anyone to read before they start making contributions.
Quirky is okay, but it's important to remember that while a Slaad's behavior sometimes seems amusing outside of the game, in-game it is not a fun experience.
Exactly what I mean. 
If you want to play completely silly - just go ahead and pull out Paranoia or TFOS. That's what those games are *for*, and they're wonderful to play if you're in the mood for them.
Far from me wanting to have it silly, it could have a bit lighter tone then original planescape. People driving cars through heavens, hells and a modernised semi-gothic bladed city while the car radio plays bach... It's not my idea of an original dark and cynical planescape atmosphere :oops:
Yeah, we really need to find a medium, not too dark and disturbing but not too funny and silly. Otherwise no one will take it seriously.
instead of silly how about a satirical tone? such as in Terry Pratchet's work, Animal Farm, Hitchhikers Guide
Too silly 
I'm a huge pratchett fan, btw.
Then again, if anyone read long dark tea-time of the soul by Douglas Adams, that might be the appropriate satyrical tone - nordian gods in everiday life
The biggest problem with taking that approach is that it's hard to write in such a way that everyone gets it. Satire is probably the second hardest style of writing there is. The hardest being horror as what's horrifying for some is just downright silly for others. It's even harder to write it in game material where you don't quite have the option of dialogue or character interaction to send things over the top.
I know I for one would have trouble telling if the setting was satire or just silly. (I have trouble even recognizing satire in the first place actually - my brain isn't wired right for it apparently.) It's also usually a lot easier as a DM, to add satire to something, than to remove it from something. I would suggest including a satirical approach under a section on how to run the game as an option - as opposed to making it the standard approach - both to keep from killing our writers, and to alllow those who want to have a serious game to have that option.
So for the game material, let's try to aim it for serious, see what we get. I've got the feeling the material itself will handle any needs for quirky that we've been wanting.
some of pratchet is rather silly but some gets to be a little darker satire. and ever hear the bit about taking the easy road? in the Old planescape it was definitly a bit satirical albet in a very dark way. if I knew how to attach sound files I'd give you a blerb from some old stuff about differant planes TSR put out.
As it is already, Modern Planescape is fairly funny in an indirect kinda way.
The Pit Fiend attorney endorses this position 
I personally favor a serious tone with a touch of the seemingly ludicrous. Tanar'ri playing rugby? That initially might sound a bit ludicrous, and it can be made so quickly, but it's easy to be serious there too.
Thankfully, I don't think we're anything close to Saturday Morning Cartoon in level. Maybe a bit Salvador Dali, or Duchamp...