Well, sound is better anyway. I've done some work on things that were pretty blatantly missing before, particularly rocket and footstep sounds. Also, all of the sounds have been replaced with some that I generated with sfxr so the only thing I'm aware of that's not free license compatible is the music. In general I like them better than the old effects anyway, especially the new autocannon sound. You can feel that one in your bones. :-)
I also spent some time on a new map that I think is kind of interesting, and as a side bonus it works reasonably well with the unintelligent bots in the game right now because there's nowhere for them to get really stuck. The screenshot attached to this post is from a very early version of the map, without final texturing or layout. This is the first really sci-fi setting in the game and I'm pretty happy with the way it turned out.
Up next is probably better bot intelligence, but that starts getting into a complex area that I don't have much experience with, so it may take a while (which is why I worked on the map first while I researched AI).