More people.
And if you want to be pessimistic, more bugs

.
Really, UU is simply Uru: Ages Beyond Myst online. No new content (save for the ability to explore the City and the 'hoods), no new story. Unlike Live, there is no content promised or even intended, and there are no plans presently to fix any bugs you may encounter (including the dreaded lag). It's simply a way for the Uru community to continue to live and breathe in the world they've been given. To keep it alive until Cyan can get the ball rolling again on the Real Deal. Hence the name

.
Pretty much the only two things you can do in UU that you can't in Uru are play Ahyoheek and scale the Gahreesen Training Wall. Ahyoheek is pretty basic, but still rather addictive (think rock-paper-scissors for 2-5 players), and also rather difficult to master. The Training Wall is like a vertically-mounted memory game where you try to scale the structure and avoid the barriers placed by your opponent. Only problem is, it's in the Gahreesen Training Center, which is hard to get to if you're bad at the jumping thing, and it also has a tendency to be rather glitchy at times.
It may not be everyone's thing, but it is at least completely free now, is open to everyone, and all of the newcomers will be on the same shard, with many of us old-timers spending a considerable amount of our playtime there as well.