Directory searching, hotkeys for everything, Calendar is more powerful AND easier to use. Searching email is faster, managing hundreds of messages easier, and the interface, snappier.

Offline is a smoother experience requiring no effort. Reminders for the Calendar actually manage to get my attention.

Rules are easier to manage, and if memory serves much more flexible.

I am not criticizing Gmail et al mind you; as I mentioned, I use them all the time. They just are more useful for personal, low-volume stuff.