I liked the startup behavior of the older versions of Thunderbird, before it had tabs. Now if you close Thunderbird with email(s) open, they will all open AGAIN when you start up Thunderbird at a later time. I don't like this behavior. I think it'd be nice to have the option to NOT have these email(s) open back up when starting Thunderbird, and have just the main Thunderbird screen open, showing the folders and such. This is my #1 gripe against the newer versions of Thunderbird.
Also, I don't like the new address book interface, and neither does a friend of mine. I think this may have been modeled after Windows Live Mail. If feasible, it might be nice to have the option of having the old or new address book interface.