Probably going to annoy someone by posting on this oldish thread, but...
I only started exploring some of the Knytt user-created worlds in about the last two weeks, so this topic interested me since I prefer the environmental levels over the challenge levels. However, I wanted to add my comment to the brief Gaia banter that was happening here.
Of the environment levels that I have played so far, Gaia was one of the ones that I enjoyed the most not just because it was large but because there were plenty of characters to talk to and places to visit. I've played a few dozen other environmental levels and you can easily create a beautiful work of art level that feels empty and/or pointless. If you're going to explore, its nice to have something beyond just scenery to visit. There needs to be things to find, characters to talk to, and preferably something to accomplish.
Gaia was nice because there were plenty of characters, lot of things happening in the background (even if you couldn't interact with it much), lots of "how do I get there? places, plenty of secret places and several endings to try to find. So more than just having a big world that looks nice, it was a big world with lots to do in it. I've played several levels that were large and good-looking, but you can only wander through so many empty tunnels, no matter how nice they look, before you stop caring.