I first got an email address with an Internet @ in it in 1986. It was [email protected], or for those of you on JANET it was [email protected] (happily I only briefly used bang paths). In 24 years I think there have been three major end-user innovations: address books, MIME and email searching.
Initially, I didn't need an email address book. Most of the people I was emailing were on the same domain (often the same machine) and so everything after the @ was irrelevant. And the number of people on email world-wide was so small that remembering their email addresses was easy (I don't mean remembering them all, just remembering the ones I needed to talk to).
And most people's domains hadn't reached the point where just using initials was unworkable. So most email addresses consisted of their initials. That made them short and rememberable. I don't recall anyone with a ridiculous address like [email protected]
But things changed: the Internet got bigger, people's addresses got more complex, I was communicating with more and more people. Hence address books.
The ability to send more than just plain text inside an email (even if it is actually being transmitted as 7-bit ASCII) was big. Prior to the introduction of MIME in 1992 there were some limited ways to send binary content in email (mostly using uuencode) but it was an ugly mess and mail clients often didn't know what to do with the contents and you were forced to save the mail to a file and manually unpack it.
Happily, MIME made that problem go away.
As email got considerably more widespread it became necessary to put it into folders to try and keep a handle on the volume. This led to the sort of trees of folders that are seen in programs like Microsoft Outlook. This is, IMHO, a less than optimal solution. The right solution is the sort of high-speed email searching offered by Google Mail. With it folders are completely irrelevant.
In fact foldering was such a pain that it was part of the reason I invented POPFile.
Two bad things have happened since I started using email: spam (first spam was in 1978 on ARPANET, but I don't recall any unwanted messages during the late 1980s at all) and HTML email. HTML email has been a spammers playground and for messages I want to receive (i.e. everything other than marketing) it's almost useless.
Minor irritations are: vacation responders, people who don't edit replies sending me gigantic threads embedded in a message.
That's one major innovation every 8 years. With Google Mail being released in 2004 we've got another 2 years to wait for the next one. What do you think it will be? For me it has to be something to do with threading. That's still pretty messy, and Google Wave doesn't seem to have improved it. I don't think the little > is cutting it anymore.