No features come to mind, but I'd really like to see the UI improved. For instance, cramping all those rows of subject lines so close without any line spacing sucks.

I tried figuring out the CSS needed to fix it, but was unable to locate any real documentation on it.

the ability to connect to 10+ IMAP accounts and not use 700MB of RAM would be a nice feature. I can either run Thunderbird or Firefox on my netbook, but not both.

I have no problem with that on this end. Perhaps it's just your install.

standard ubuntu builds, across multiple computers. Perhaps not.