StarCraft 2 has tastefully upgraded the UI so that you can do lots of things with less clicks than before in SC 1. (Of course, that means at the higher end, people will just do more now than they did. But it's quite a nice feeling on the beginner level, too. E.g. you can get a worker to work right after it's build. And there's also a button for finding idle workers.)
Honestly, the graphics may be better, there may be new units, and balance may have improved or whatever else, but the UI improvements are the thing I love the most.
