1upyours has so much awesome info on this.
The one bad things I heard about:
-You must play the first level before you can create things. To unlock tools/objects in the create mode, you must get them in the single player game.
-The more people creating a level together, the more slowdown there will be (very little with 2, a good amount with 4, doesn't seem like any with one player). Doesn't seem like any when just playing the levels.
-screen scales in size for more players, so if someone gets left behind, it is seemingly random with who gets left behind and who respawns (this is local multiplayer).
All good:
-Single player is supposedly awesome
-amazing music
-narration from someone in Hitchhikers Guide to the Galaxy
-single player is actually difficult and not a push over
-able to make multiple finish points in a level (think Super Mario World)
-the sharing interface basically are planets, each planet is a user, on that planet you can find their various levels. In the beta there are 25 slots.
-MM is not going to stop supporting this game when it is released. They aren't going to go and start on LBP2 or another game, instead they will continue to treat LBP as their main game.