When Gmail first came out, I certainly would have agreed with that statement. When I first received a Gmail invite, I placed "make Gmail address main one" somewhere on my to-do list.
Since then, however, Yahoo has mostly-replaced Yahoo! Mail with Yahoo! Mail Beta, which I strongly feel is far superior to Gmail (and far, far, far superior to the Yahoo! Mail Classic). Its main advantage (but definitely not its only) is its GUI, which contains its own tabbing system, the ability to read an e-mail in an iframe merely by selecting it in the inbox, the ability to move between folders via drag-and-drop, and much more.
http://en.wikipedia.org/wiki/Yahoo%21_Mail#Yahoo.21_Mail_AJA...
Relatedly, but off topic: It still appals me how many sites don't gracefully degrade back to vanilla HTML. Or even check, for heaven's sake, merrily making all of their links Javascript-only. Not nearly as many as assume that you can't possibly be using your own fonts or setting minimum point sizes, admittedly (my eyesight isn't what it used to be, and it never used to be up to much) but even so... I keep hoping that someday a revision of HTML will come out that banishes the javascript: protocol from links altogether.
I use 3 different computers running two OS'es - that is the only reason I use web-based mail. Back in the day when everything was on my laptop, I liked "TheBat!" and ThunderBird.
I use evolution for work email and use its "vfolders" extensively. They have a different name, and they predate "tagging", but they're really the same thing, and difficult to live without.
In gmail, I would prefer proper threads instead of "conversations", but in practice the difference is minimal.
The crappy editor is its biggest problem, but even that can be worked around.