I saw the regular version (not 3-D). As several friends said "It wasn't as bad as I had expected." Most of what I noticed reflects what some have already said. It's a fun, visual fest with great design and a good soundtrack. Go into it with that expectation, just enjoy the ride without thinking about it too much, and you'll have a good time. Expect more -- like, say, a story -- and you'll be disappointed.
there aren't any real plot spoilers below, but a few visual spoilers...
Bonus points: some fun use of capoeira in the early gaming fight scenes, along with a parkour-influenced use of 3-D fighting space by Flynn. The 2-D game grid from the first film has been updated to a cool multi-tiered gaming space, which I loved. And some beautiful air fight scenes toward the end, the electronic trails are really cool. Oh, and Cillian Murphy was great, but also wasted in such a small role. I hope he at least has a larger one in the sequels.
Shortcomings: The science/tech made no sense (I won't even list all the inconsistencies and WTF moments), many of the characters were 2-dimensional and as a result their actions seemed contrived, a lot of it was predictable, and yes, it directly lifted from many classic sci-fi films (including the last Matrix film, as someone mentioned). ETA: oh, how could I forget! I wanted to sing Welcome to the Uncanny Valley every time CLU is on the screen. One of my friends found it so distracting it became irritating for him to try to watch it. I think Tron: Legacy is now synonymous with Uncanny Valley and all the pitfalls of it. ...On a separate note, if you're a mystic warrior with the sum of all knowledge, you'd be smart enough not to wear 5 inch heels to do battle.
Another bummer: I was disappointed that there was only 1 female character for every 50-100 male characters, literally. I saw a total of 6 female characters in the whole film, including Quorra -- of those 5 didn't have more than 2 minutes of screen time and little to no dialog, 4 of those were never seen again, and 2 were twins with no dialog so they counted as 1. There were so many opportunities to have included some females in what is a major sci-fi film. Any of the programs that were "arrested", any of the gaming competitors, any of the clones, any of the henchmen. Blah.
I guess in The Grid there are no women, like in space there is no underwear.