Off the top of my head - offline support, PGP support, multi-account support, decent mail-list/threading support, separation of client from service provider..
I'm actually quite a fan of the gmail interface; it is one of the most usable webmail experiences in my opinion (I use google apps both at work and for personal use). For day-long usage, I'd rather stick with mutt/vim.
It has labels, of which you can apply one or more of to a message. When represented in IMAP, it appears as folders (though you get duplicate mail objects for every additional folder that a mail is tagged with).
If you only apply only one label to a mail at a given time, it works as a folder would do just fine.
I'm actually quite a fan of the gmail interface; it is one of the most usable webmail experiences in my opinion (I use google apps both at work and for personal use). For day-long usage, I'd rather stick with mutt/vim.