The sky could have used more stuff. The number of islands was fantastic; it's just that most of them were so barren. I can think of like 5 islands that actually had a good amount of stuff on them; the Pumpkin Island, the island with the bamboo minigame, the bug hunting island, Island of Songs, and probably one more that I'm forgetting about.
Skyloft was great, though. The townspeople were likable for the most part and it was pretty big. Shame you can't land in more places after skydiving, I hate how there's so few spawn points there. =/