But one thing that gets me is the threaded messaging system.
I don't mind the concept and see where it can be useful. But in practice it can be incredibly confusing and difficult to find individual email messages.
Gmail's groups functionality can be useful, however Gmail isn't a discussion forum or mailing list, it is first and foremost a personal email application. Trying to be everything isn't always the best way to go.
I've also found that it can sometimes also cause confusing situations of replying to the wrong person because you've picked a message out of a thread. I've found the confusion gets worst when you're switching from using Gmail in groups to sending and receiving individual emails.
But its not a mode and I'm yet to find a way to turn this feature off ... there is a Google groups thread about it here ... perhaps there is a better email service that is not as confusing !