Personally I'd like to see the chat client, although Zoop brings up an excellent point. But, I post and chat on IRC at the same time (heck, I'm doing it right now) so maybe it wouldn't be such an issue.
After that personally I would say the CDN. Speed isn't really an issue for me, but less server load is always a good thing.
Zoop also brings up a good point about the gallery in that it's something visible, which is good because then people actually see the effect of donations. Still, what with free image-uploading sites (and the fact I have my own space) this isn't really a big draw for me.
A custom skin would be nice, but I honestly like the one we have now. It can be lived with at the very least. Actually, speaking of which, has it disappeared for anyone else? It's suddenly the default for me and I can't seem to change it back.
An upgrade to run multiplayer games doesn't really interest me at all, personally. But it could be a good idea if people are interested in it. Good for running the MC server you have as well, I suppose.
So, there's my two cents. Don't spend them both in one place.