1. Keyboard short-cuts.
2. One login info - Gmail - lets you into Blogger, AdWords, Calendar, Docs & Spreadsheets, Picasa, News, etc etc. Basically it's like a master key to all things Google.
3. After integrating Better Gmail hack, it turns into a super-account with. Labels, colors, etc. My favorite is the Google Reader's addition on the same page. Switching back and forth between mail and Reader is a breeze.
4. Email threads. Frustration turns into Roberto Benigni during the Academy Awards.
5. Excellent spam control. Which means excellent mental health - because no email subject keeps reminding me my 'member' of the parliament will never be the 'president' of her 'country'. Or I can buy Ado8e V8 for only $80. Didn't know Ado8e made tomato juice.
6. Lazy clicker's delight! Now you don't have to open the mail to read its contents. Simply right-click on one of the email list and a pop-up preview will appear with additional options.
7. Accidental Humor. By that I mean when you open an email where a beloved may have written: 'Baby, I miss you. Have you eaten? Wish North Pole wasn't this cold,' Gmail's sponsored text ads on the right will show: "Don't miss Christmas sale on baby food. Target.com" which would be right above the link 'AreYouASlackerMom.com'
I think 7 should do for now. What if Google did a Trends option on your email? It could find out what we've been obsessing about, what words we use more often, a chart of our moods over the months, who we're gossiping about etc. Someday, large file transfers via GTalk, have GrandCentral integrated video/audio messaging and calling, wire money, etc etc.
In short, and I realize I sound like a broken re...scratching disc...Your Gmail is not just an email account. It's a master key to many, many doors.