Among the many problems I have with the Microsoft suite, Outlook is at the top of my list for generating the most frustration. GMail's priority inbox and new-style inbox where machine learning is used to sort out mail is a productivity booster for me. With Outlook, I regularly lose important mail because it is buried under the difficult to filter masses of other email.
I could go on about how the lack of robust collaboration facilities are a direct time waster for my team, but I don't want to hijack this thread further...