Lots of people are using web-based email like Gmail these days. Most of my email is short, quick stuff but sometimes I write longer, more thoughtful pieces—mostly at work (we use Gmail). Later, in other longer emails I might want to reference the previous thoughtful piece. To do that, I need to actually copy and paste it in the new email.
What if email had permalinks? Then I could just reference my other email with a link similar to how a wiki works. In fact, it occurs to me that variations on email—giving it powers of blogging, wiki, IM (already happening in Gmail), voicemail, and a version that's just out there making anything possible could be useful going forward.