If this is more of a mmorpg than a sandbox, And has physics comparable with Gmod, Expect me to attempt to become the first lego man in space in LU :D
I don't think it would do as well if it was purely an MMORPG. Let's think for a second, why do kids
and some adults :P play with Lego? Because you can build things you came up with in your head. If Lego wants to make an RPG, they should either quit right now, or go for the sandbox style, because if someone wants to play an MMORPG, they play Wow, or if they're eight, Runescape.
As for my views on LU; there can be many ways to play a game on a computer. The first being played in your Internet Browser, the second being downloaded straight from the 'net, like BL or bought from a store and stored on a CD. If it was played in an internet browser, then it would lag horribly if it wasn't done right. Blockland proved to me that a game from the 'net wouldn't lag most of the time. As for a CD approach, this could be best. However, if you're scared of little kids being the primary consumer, then this should be avoided. Only people who are interested in the game enough would try to find it, whereas little kids won't. Besides from what I've seen, it could be fun, what with the sheer Lego style and humor.