Our biggest problem was the lack of single-sign-on support. Perhaps just our configuration was wrong. But users are to spoiled to (after logging into their workstation) have to log in again to get mail, and again to look at their calendar, and again to access their contacts. Many users would have a real problem with that.
Also, how can you archive mail (like you can move mail to pst files in Outlook)? Without those two features it would be unworkable for us.