I'd like to see dataloss and corruption bugs addressed at a high priority.
Optional GMail and iTunes layout sounds very interesting to me. There is a screenshot as well.
How about a new theme to match that of Firefox 2.0
Conversation scoring
Having a nice UI for scoring messages and conversations would be nice.
For instance, I'd like to be able to say "messages in this folder that match these filtering rules give a +5 bonus to the conversation they are in". Or "conversations I've participated in get a +10 bonus", etc.