1. Redesigned chat interface. One of the biggest issues I have on a regular basis is the clunky chat interface. What happened to the original battle.net chat? When I see what my friends that play wow can do with their unaltered chat windows I tend to get a little jealous. I would like to be able to easily whisper multiple people, create chat channels, party invites, etc. all through the chat interface. I have been able to do this in previous blizzard games, why not now?
2. More community portals on the main launch page. Riot games does a fantastic job advertising major tournaments and community news through the league of legends client. Couldn't we do something similar?
3. SC2 is a hard game. If you just try to pick it up on your own merits it could be easy to become frustrated and stop playing. Articles from sites like Team Liquid could help educate the community instead of keeping up the same few news posts for months at a time.
4. Clan support. Why exactly can't we get clan tags much in the same way guilds have guild tags?
5. Multiplayer replays would be really nice.
6. Get rid of the popularity system for custom games! It makes it extremely hard for new games to become popular without literally an entire website backing and playing a map till its at the top. The mod community has literally been RUINED from wc3 and scbw! With the amount of time and effort that went into creating the blizzard map editor for starcraft 2 it sure isn't being utilized for making good custom UMS games.
This is by no means a definitive list, but I feel many people would agree these would all be positive changes for the game and community.
7. Tournaments run through the client like in war 3.
8. A tournament server that large events could utilize to try and combat latency and BM.
These changes are wishful thinking, but would make me very happy.