Small things (which I'm sure will be fixed in the future) still need work.
One such small thing is the toolbars ... they are not as flexible as IE's toolbars. For one, you can't move them around, so if I wanted to put the address bar next to the menus, tough. that was one of my main ways of gaining real estate. I downloaded Mozilla's version of the google toolbar (because the built in one kind of sucks) ... it works great, but it's taking up a whole line when I would have put it next to the menus.
That being said, I still haven't found anything to complain about with Thunderbird. It kicks arse.