As a daily user my personal preference almost align with their goals. I look forward very much to the day that I can stop using mail-notification in Gnome. The Thunderbird built-in system has seemingly broken polling feature (not sure if it does anything when running minimized), and has no option to specify folders that you want fast notification when a new mail arrives. I always miss Gnome mail-notification when I am using Windows, and sometimes have my laptop running Gnome next to the Windows computer just for this purpose.
The second feature I would very much like them to work on is search. It is horrifically slow, seems to miss some mail (html only maybe?) and the result window has terrible UI. You can't preview why the search found something so either you run a more narrow search or manually go through each email it found. There is also no threading in the result window so you lose a lot of context.