Lanefan
Victoria Rules
Not quite everything else. The spells were a real mix - some were crazy powerful, others were decidedly underwhelming to say the least. Ditto for the new magic items. And some other bits were in fact really good, or had the germ of something good in them.The thief-acrobat was one of those concepts that sounded great, and you wanted to be great, but was terrible.
Weirdly, everything else in UA was total power creep, but the T-A was actually underpowered, and considering it was based on the Thief ...
I remember when UA first came out I and another DM went through it and did some judicious pruning. We kitbashed and adopted Cavalier but not the other classes (by this point Barbarian was already a Human sub-race in our games anyway). We adopted maybe 1/3 of the spells, some of them quite modified; and some of the magic items as well. We ditched Comeliness and the new roll-up methods. We significantly toned down weapon specialization but did adopt it.
The thing we took from UA that I think was the best addition to the game was the Cavaliers' percentile-increment system for stats: we gave this to all classes and still use it now.