The others have pretty much covered it all.
1. Search could do with some improvement (e.g. only include answered threads).
2. "Recent Content"/"Recent Activity" sections on the front page. This is useful to see if anyone is having issues and to discover new ways of doings things with the modules but...
a) I'd like to filter out specific types of content (e.g. filter out voting and bookmarking - when people go and vote in all the polls this is just full of poll threads)
b) I'm not sure if it's just me but I only get a page or two of recent content. Due to time zones, I typically visit thwack near US close of business time so only see a fraction of what may have been posted. This means I open up about 20-30+ tabs of all the sub-forums to see what has been posted (which also means I don't get to do it often since it's a massive time sink...).
I don't know a lot about using RSS feeds but I'm thinking of giving them a shot - anyone else using them or can suggest a decent reader?