LEGO Universe has been somewhat thin as of late, but that's beginning to change. Over at 1up you'll find a meaty preview that covers quite a bit of ground, including basic MMO features like progression to LEGO specific stuff like building quests. The building mode is of course the keystone to this game, and the fact that a player cannot be harmed when entering the mode speaks volumes about its importance.
Aside from building mode, zones in LEGO Universe are likely to be distinctly memorable due to the licenses' ability to have pirate, ninja, robot or even Star Wars themed planets -- assuming they're capable of using any of their real world toy lines within the game. And as for combat? If you've played any of the console LEGO games, then you'll know what to expect, which is essentially action-based combat. There's a lot more detail in the actual preview, so check it out if building with bricks is how you get your kicks.