I got interested in the Ptolus book because of the larger world, which I think is great, and encompasses everything from Arabian Nights to fall of Rome stuff. Heck, you could even file the name off of Nyambe and insert it into Praemal (there's a part of the Distant South that seems African).
The planar stuff is an issue, I agree, but Monte already stretches it in the Big Book with demiplanes reachable from Ptolus, the Ethereal Plane/Sea and even the Plane of Shadow. You just can't go out from there into the rest of the multiverse. That said, his campaign diaries lay out exactly how to knock down that wall and quite a bit of the book focuses on various in-the-know bad guys working on escaping Ptolus.
The creator god has also made at least one exception in the past, so it's not like there's NO way out of Praemal, it's just very, very difficult.
The planar thing was the one thing that made me consider not using the world for my campaign, but in the end, I decided that the Plane of Mirrors (which I really wanted to use) would fit in just fine, I just wouldn't create any mirror sets that had connections outside of Praemal, making it an extradimensional tunnel through the world and not much of a different plane, in effect.
Within Ptolus itself, there's a pretty good cultural diversity, since this is a Sanctuary-type city at the edge of a (falling) empire, full of people who don't fit in with everyone else, either because they're the loser in a political struggle, or they're criminals, or they're non-humans, or because they're Evil or Good and attracted to the badness in and around the Banewarrens.
There's even a mechanism in the game to allow "good" monstrous humanoid characters, although that doesn't necessarily ensure the characters won't get a lot of grief (including not being protected by many Imperial laws) until they're recognizable celebrities. (If I were to play a kobold in Ptolus, I'd likely want to take a lot of ranks in Disguise.)