Planewalking

Which is one of the things I disagree with 3E about. If you think about it, it makes no sense to walk from Acheron to Baator. After all, where do the cubes stop and the fiery plains start? Similarly, Carceri causes problems from either the Abyss or the Gray Waste. Travelling from Arcadia to Mechanus, or Bytopia to anywhere ("where did the ceiling go?") all require a fundamental change in planar topology.

I think the Great Road refers to portals.

Nevertheless, I always thought those "thin places" were a good idea. If the river styx can connect the lower planes, and the Oceanus can flow through the uppers, why shouldn't there other, similar places with less power? There could be a forest on Ysgard, where those who get lost end up in Arborea. Or a deep cave on the plain of infinite portals could connect to Pandemonium. I used places like these, but only rarely.

The Great Road refers to the network of portals connecting the Outer Planes. This was explained in the Planescape Campaign Setting boxed set.

The modrons march through portals in the various gate towns. Their ordinary route is described in The Great Modron March. Not surprisingly, it resembles a gear, clockwise across the Great Ring, alternating Outlands portals and portals on the Ring.

1. Beginning in Regulus, take Automata portal into the Outlands. 2. Cross the Outlands to Fortitude. Take Fortitude's portal to Arcadia. 3. Cross Arcadia to take a portal (which appears as a white trellis) directly to Mount Celestia (Mount Celestia's gate-town to Arcadia is called Nemmiron). 4. From Mount Celestia, take Excelsior portal (a bobbing light above the ocean that you have to chase down) to the Outlands. 5. Cross the Outlands to Tradegate. Negotiate with the Master Trader to get into Bytopia. 6. Cross Bytopia to find a portal (which appears as a cavern with radiating lines shining within) leading directly to Elysium. 7. From Elysium, take the Ecstasy portal (a day's travel upstream from Release-From-Care, appearing as a dark cavern on the bank of the River Oceanus) into the Outlands, so the modrons come out from the Ivory Plinth in the center of town. 8. Cross the Outlands to Faunel. Negotiate with Wrath to get into the Beastlands. 9. Cross the Beastlands to find the Roaring Gate leading to Arborea. 10. From Arborea, use the portal to Sylvania in the center of the town of Thrassos. 11. Cross the Outlands from Sylvania to Glorium. Either the root of Yggdrasil in the caves near town or the Maelstrom portal in Glorium's fjord will work. 12. Cross Ysgard to find the portal to Limbo, which takes the form of a well (Yggdrasil also leads to Limbo, in the island of Pinwheel). Olidammara's realm might have a portal to Limbo in it. 13. The portal from Limbo to Xaos is an extremely chaotic region called the Immeasurable. It constantly changes in appearance on both sides. 14. March across the Outlands from Xaos to Bedlam. Use one of the portals on the base of the great hill Maraush to enter the Howling Plane. 15. From Pandemonium, the portal to the Abyss looks like a flat red square on the side of a cavern. The modrons come up from a black pit like those that lead to deeper layers. 16. Take the portal beneath Broken Reach in the Abyss to Plague-Mort. March across the Outlands to Curst. 17. Take the Razorvine Gate from Curst to Carceri. 18. The portal from Carceri to the Gray Waste looks like an obelisk carved with tortured faces. It is taller than it is wide. 19. The portal from the Gray Waste to Hopeless looks like a spinning silver coin, most likely protected by a fiendish fortress. They come out from a black pit filled with a tarlike substance. 20. March across the Outlands from Hopeless to Torch. 21. There's said to be a portal from Torch to Gehenna hidden in the swamps. This seems more practical than the main gate, the great blood-red orb you need to fly to get through safely. The modrons come up in Gehenna from a seemingly bottomless chasm in a cavern floor beneath the surface of the plane. 22. The portal from Gehenna to Baator is another chasm in a subterranean chasm floor. They enter Baator through a flaming hoop protected by amnizu and abishai. 23. The portal from Baator to Ribcage is in the town of Darkspine. It should be another flaming hoop. 24. They march from Ribcage to Rigus. The Lion's Gate in Rigus leads to the Blue Cube, the Battle Cube, or Resounding Thunder. They enter the plane from what appears to be a silent sphere on the surface of one of the cubes. 25. Resounding Thunder has the portal to Mechanus in it, in the town of Nihao. Nihao leads directly to Regulus, and they're done. In the most recent march, they took a detour, using a portal in the goblin city of Gashmog in Clangor to get to the duergar realm of Hammergrim, through another portal to the Mines of Marsellin, and finally through a portal to Mechanus there.

They can lead to Athas, it's just that portals to isolated worlds happen to be rarer.

And there happens to be an Athasian Halfling ghetto mentioned in one of the official products too.

There's also New Tyr, a community of Athasian expatriates in the Hive Ward. There's also a portal to Athas in Pelion, Arborea's first layer (and Athasian elves have been seen in the desert there), and Athas has vortices to the Inner Planes as usual (though they have different names for the paraelemental planes there).

But sure, there's no reason you couldn't find a portal in Sigil leading to Ravenloft, Agathion, Eberron, the Far Realm, our modern Earth, the Demiplane of Imprisonment, alternate-reality versions of Sigil, the past or future... absolutely anywhere.

Yea. conduits were droped from 3e. For simplicity's sake. In 1e & 2e there were several types of planar connections.

-Conduits connect the Prime with the outer planes. They cross the Astral, though the trip is so fast few travelers notice. However, it is possible to fall out of a conduit and end up on the astral. It is also possible for the end of a conduit to change location, dumping a traveler somewhere they didn't expect.

-Vortecies link the Prime with the inner planes. They are usually open 100% of the time, so exist in areas with a serious environmental connection to their destination, i.e. a vortex leading to the elemental plane of fire or para-elemental plane of magma will be in the middle of a volcano, one leading to the plane of water will be in a large water body, etc.

-Color Pools exist on the Astral, and lead to the outer planes. There is some debate, depending on the source, as to whether one can tell the destination by the color. In any case, they are only colored on one side, so a traveler can blunder into one without ever seeing it.

- Portals and gates are similar in that they lead from one plane to another, or from Sigil to somewhere, without passing through anywhere else inbetween. The major differance is that gates are perminant and constant.

Of course I may be mis-remembering some details, but I'm sure some master of canon or another will correct anything I left out.

Well, that depends.  To a planar scholar, the differance is huge: they're different, period.  To the average planewalker, the difference is academic ... until a conduit drops one off in the middle of the Astral, far too close for comfort to a gityanki fort.

Personally, I like the idea of various types of planar connections.  They give scholarly NPCs something to rattle their bone-boxes about.  They add to the complexity, and consequently the wonder and mystery, of the planes.

So, in my campaigns I use Portals, Gates, Conduits, and Votecies all.  I also use Planar Barriers, large-scale regions like the barrier between the layers of Bitopia, and Conjuctions (from Beyond Countless Doorways), which are when planes temporarily overlap.

Another interesting thing to keep in mind about conduits is they're the channels through which divine power passes from power to priest and vice versa (in the case of worshipping).  They're also the means by which souls leave the Prime and journey to the Outer Planes.