Ok, I'm way late to the party here, but feel the need to share. I am one of those who saw the original in theaters and loved it, and have watched it several times through the years. I went to see Legacy and pretty muchly hated it. Yes, it had awesome special effects (for the most part), and was a sleek-looking movie. Setting the effects and eye-candy aside, there are several weaknesses that undermine its relationship to the first movie.
Tron had a unique plot, Legacy has a Matrix-meets-Lawnmower Man plot.
Tron had (mostly) likable good guys, I had to work at liking everybody except Alan. Quorra was nice to look at, but didn't really add anything at all.
Tron had some great bad guys - you just love to hate Sark and the MCP. While the concept of Clu being a bad guy is great, the execution sucked due to the limited emotional range of Jeff Bridge's artificial face. And you never get to see Tron's face at all. Even Darth Vader has a facelike mask.
Legacy is a dark, joyless movie, with some attempts at sly humor; whereas Tron had a great sense of honest humor (remember Bit?).
Tron had a some great spiritual undertones, with the good guys being persecuted for believing in their Users/Creators (even tho the bad guys knew they existed too), and a Christlike sacrifice by Flynn. Legacy has more of a Zen thing going on, the yin-yang being Clu and The Dude.
There is so much more - blood in the computer world, the annoyingness of Zuse, etc. etc. etc., but I think you get how disappointed I am with Legacy compared to Tron.
My $.02.