This is a 'meta-issue' for linking all issues related to advanced email support. That is, towards being able to use email as a primary JIRA interface – ideally, not even be aware that JIRA is involved.
The biggest use-case I have is public opensource projects. All communication happens over email, and there is very little distinction between an email and an issue, neither conceptually nor physically (the tracker is configured to send emails for new issues/comments). Frequently, long conversations are held via comments in an issue 
For established projects, a significant proportion (~40%) of non-cvs emails are generated by the issue tracker. It would be a massive timesaver if developers didn't need to run off to a web interface to comment on issues.
JIRA has some rudimentary support for this (CreateOrCommentHandler), but JIRA-generated emails:
- do not support threading (
- are IMHO pretty ugly and not easy to scan quickly compared to Bugzilla's. (
- do not customize the From: address to indicate who the commenter is. (
So I think getting decent email support would be a fantastic way of capturing the lucrative open-source market. More than any other feature, it would leapfrog JIRA over its competitors.
If JIRA's email support was enhanced sufficiently, it could also be used as a support system backend, tracking support requests, not least here at Atlassian. This is a whole new market, probably justifying a new product stream, but I think the enhancements mentioned above would give us an 80:20 solution.
 For example, see some of Mozilla's longer threads, like http://bugzilla.mozilla.org/show_bug.cgi?id=97284 .