My least favorite part of Legacy is how it looks.
I watched the original TRON for the first time before watching Legacy, and despite being 30 years older, its visuals are far more interesting and fitting for the world presented. Legacy is a generic dark cyberpunk city with orange and blue neon everywhere; instead of capitalizing on the unique setting of the Grid, Legacy turned it into something snoozeworthy. It doesn't help so many of Legacy's visuals are very obviously inspired by the Matrix sequels.
Just compare how the people in the Grid look, or compare the MCP to CGI Jeff Bridges and evil biker minion. Sure, the 30-year old counterparts might look a bit hokey today, but they also show a lot more imagination.
The plot and characters are terrible too, but that's more understandable when the original TRON was bland outside its visuals. I wouldn't rewatch either movie, but at least I appreciated the first TRON from a visual perspective. Legacy just makes me mad.