We had been using RT/WebRT, but found that after >400 tickets, the
system began to slow to a crawl (it doesn't appear that JIRA suffers from this problem, though). We are a small company with 2 people (also software engineers) doing the bulk of the support work. So when I think of a support tracker for our use, I imagine it having, as you say, a pretty simple feature set:

• Ability to receive external e-mail, create a ticket, and have
  certain comments (correspondence) sent back to the originator/
  requestee.
• Status per ticket
• Owner per ticket
• Attachment handling is nice also.

We don't really deal with any notions of priority and/or escalation of
support tickets here, as there is usually only a small amount of traffic. I should add that I would like these support tickets to be
'interchangable' with normal issues in JIRA. They might be created in a default project, but could be moved to other projects and still maintain the requestee, etc.

In the future I can imagine us exposing JIRA to our customers and
importing a customer username/password database already in use, but for now we are mostly interested in e-mail support management.