I saw it about a week ago. Overall, my biggest impression was one of missed potential.
(Note, here I'm talking primarily about the story and the world-building, not about the cinematography.)
The overall structure was a weakness from the start. Sam Flynn turns out to be yet another Prince Harry character: the heir to the throne who goofs around and avoids his inherited position until he's handed a confrontation that forces him to prove himself, at which point he rises to the occasion as a True Prince. We've seen this before; it's the usual aristocratic nonsense: worth is not achieved, but inherited and then revealed.
Contrast the original: Kevin Flynn was an honest working hacker who was forced to go rogue when he was screwed over by a yuppie coworker. Kevin's triumph was to prove himself as a creator. He set out with the aim of showing that he and not Ed Dillinger was the author of Space Paranoids; and in the end, he accomplished that goal, but in a way that -- through his creative "User power" -- changed the Programs' world for the better.
Sam isn't a creator. He sets out with no particular goals of his own; he is handed all his goals by his inheritance. Kevin Flynn was a creative adult seeking justice; Sam Flynn is an irresponsible rich boy growing up. And that's a story that's been played out far too many times.
One of Legacy's few big world-building ideas is the emergence of the Isos: Programs evolved from the System itself, rather than being created in the image of a User. This could have been huge. But instead it is presented merely to give Sam's love interest a tragic backstory. The war is over; the Isos lost, here's the last surviving princess of a dead race. Give her a hug.
The political vision of the System in Tron is more complex. There are old powers in the System that defy the MCP's regime at personal risk to themselves: Dumont at the I/O Tower. The MCP's assimilation of the whole System into itself is not complete; it can be resisted. In Legacy, CLU's genocide of the Isos is over and done with
Another new world-building idea is the possibility that a Program could use the laser terminal to escape into the real world: that the laser wasn't limited to objects that originated in the real world (oranges or Kevin Flynns), but could also play back a Program into human form. Thus Quorra's escape; thus CLU's threat to invade our world with armies of Programs.
Well, Tron's MCP didn't need armies to take over the world. The MCP could just hack the Pentagon. In Tron, the deep entanglement of the real world and the System is made clear: the MCP can threaten Dillinger not with armies materializing in ENCOM's laser bay, but with the legal and political forces native to our world.
Ironically enough, the 1982 vision has more in common with today's Internet-enabled reality than the 2010 version. As far as we know, the System in Legacy isn't even on the Net: it's a dusty minicomputer sitting in the basement of Flynn's Arcade with barely enough connectivity to reach Alan Bradley's pager.
Ultimately, CLU is much less of a real-world threat than the MCP. The MCP had taken over the System that ENCOM used to do its business, and was extending tentacles into banks, major governments, and who knows what else. CLU's domain is that one minicomputer; the big threat would be shut off if Alan or Sam had just unplugged the laser terminal.
Both of the above two problems point at a bigger problem with Legacy: it ultimately doesn't take Programs and the System seriously as an independent sort of intelligent existence rather than a mere imitation of our world.
Quorra longs to see the sun; CLU wants to get out into our world to "perfect" it; the Programs have nightclubs and sports arenas imitating human ones. The way it's presented in Legacy, the best thing that could happen to a Program is to get out of the confining, artificial System into the authentic, sun-blessed, material world.
That notion is alien to the original. Tron, Yori, and Dumont may revere the Users but they don't want to become Users. They want to free their own world and live in it pursuing their own purposes -- not escape into the human world. They aren't imitation humans who want to grow up to be Real Boys like Pinocchio -- they're Programs, and they know what their purpose is in life: it's to fulfill the goals their Users set up for them.
(Extra bonus for real sf nerds: Tron's Programs may have something in common with C. J. Cherryh's azi: confidence of purpose. As Grant would put it, self-doubt is for born-men. Azi do not wish they were born-men; azi take refuge in the certainty that born-men lack.)
And speaking of lost story potential, how about Rinzler? Anyone who'd seen the original knows that Rinzler is a hacked-up copy of Tron from his very first appearance, thanks to the "T" insignia on his chest. Kevin Flynn mentions it once in passing, and at the end it's clear that Rinzler is "rebooting" back into Tron. But Rinzler hasn't had enough character development for us to care: he's a literally faceless killing machine. And as killing machines go, he's got less character than Darth Maul, and that's saying something.
All in all, Legacy came across to me as too circumscribed of a world, and Sam Flynn as too much of a True Prince cardboard character. Movie-wise, I wanted to see more of the Isos and a lot less of Dr. Frank-N-Furter.