Here's a quick Gmail goodness grab-bag top-10...
- Spam filtering. It just works. I estimate it kills more than 99% of my spam, and the only occasional false positives I get are from Yahoo Groups (which is a spam cesspit anyway) and mailing lists that include spam samples (uhhh...)
- IMAP access. Yay, we've been asking and asking and asking for it, but it finally arrived yesterday.
- Local front-end servers. Recently, Google moved the POP/IMAP/SMTP servers I connect to. They're not now in the U.S., but much closer to me (in the UK?). Some sort of routing cleverness, I dare say. This means downloading a load of messages is now very, very fast.
- Search and Filters. Fast, flexible, frequently-very-useful. Especially when combined with the saved search extension for Firefox (using Greasemonkey or the Better Gmail extension).
- Labels. I know some people hate 'em, to which I say, "Just think of them as folders." But they're so much better than folders, mainly 'cos you can "file" a message in more than one of them.
- Fetchmail. Integrated, as way of grabbing your email from other accounts, using POP. Saves auto-forwarding, which is increasingly broken.
- AJAX. Not as ground-breaking as OWA, not as flashy as Oddpost/Yahoo/SWA, not as mashable as Zimbra, but fast and usable all the same.
- Keyboard shortcuts. A big productivity saver. I hate to move my hands off the keyboard to find my mouse -- that's a key reason why I don't "do" Mac OS.
- Google Apps. A white label version of Gmail is included in Google's hosted applications service.
- Free. Yes, as a confirmed cheapskate, this is a good thing. Even Google Apps is free for up to 50 mailboxes. No more do vanity domain owners have to suffer the slings and arrows of outrageous email forwarders.