I realized that I needed to finish the first pack of missions in Little Big Planet 2 story mode to play the community levels. Now I would like to know what else I get if I finish the other missions, and the whole game.
You get a pin and trophy for completing the main path levels in each world (i.e. challenge levels don't count). When you complete all the main path levels in the game, you get another pin and trophy. That's it.
Of course, along the way, you collect a lot of prize bubbles, both in the game and for completing each level. These help when creating your own levels.