With the rollout of "Gmail 2.0" (which I have yet to get on my accounts), Google Operating System did some digging in the code and found that quite a few more unannounced features are likely coming beyond what have already been noted.

They point out references to:

Colored Labels. Which would be a HUGE help for me in sorting through my mail.

Detaching Messages From a Conversation Thread. Again, great for the times you want to star a certain part of a long back-and-forth but not have to read through the entire thing to find it.

Jabber Transports. If done correctly this greatly expand the use of Google Talk within Gmail and other Google properties, which is clearly something Google is going to want for their OpenSocial push.