threading killer
I'm not saying whether I hate threading or not there, but there is one killer. People often pick a random email and reply to that rather than creating a new one for a new topic. Going through emails randomly to pick out requirements/details/etc is time consuming.
The ability to order things in the way that helps me to do something (e.g. by date) is a fairly fundamental requirement of pretty much any UI. Anything that forces a one size fits all is pretty poor, give me something that I can choose that best meets my needs at the time anyday.