Outlook :'(
I think it's because Thunderbird is very buggy when it comes to RSS feeds. If my PC happens to crash when TB is open all my feeds become corrupt and it will download all the feeds again, and again and again.
Also I would like to see more features like have it running in the background. Faster IMAP etc. I know there are extensions out there, but some things need to be built in.
Just my 2 pence anyway.