Multiple account, huge storage of every email in a single SQLite database that you can prune at will and that can do in-browser full-text "as-you-type" search over the entire archive without even struggling.
Currently have all my emails for about 10 accounts going back to, the earliest, 2002 - and that's only because before then I wasn't sucking stuff into Opera or didn't keep emails. I have another computer somewhere with everything pre-2002 stored in UNIX mailbox format that I could import if I could be bothered.
Tagging, Bayesian spam-filtering, insta-searches over Gigabytes of email through multiple accounts, IMAP integration (folders, tagging, etc.) and a lovely interface to browse just one account, just unread, just ones that fit a criteria, etc. without having to work out how to do it.
But, saying that, haven't needed to bring pre-2002 archives out in the last ten years except maybe once to get some ancient password that I could have just re-issued if necessary anyway.