With all the hype that was around it, I got myself a GMail account some time ago, when they were rather "rare". While the UI is still very good for a webmail, it has also a lot of things I do not like.

I don't use it heavily - not for personal emails, but only to recieve some mailing lists I don't want to clutter my regular mail (and there are no privacy issues there... Google will index them in the list archives anyway), and to recieve some off-site notifications by some servers (it's always better to get warning emails etc. off-site so you can still access them when the site is down... oh, and if you have an email relay, try to setup a direct transport for your email, so it works if there is something wrong with your email relay, too!)

My top issues with gmail are:

• Functions moving their location in the menus. E.g. when I empty my spam folder I always try to pick "delete" from the dropdown. But while the "delete" function is there in every other, it's not there in the Trash and Spam folders (instead it's a big button where the "Archive" button usually is).
• "Delete" is hidden away. GMail assumes that you don't want to delete any mail. But I do. There are so many mails not worth keeping at all! Especially with the logcheck mails I get - sometimes one every hour - I really want to delete them permanently. It's just of no use to keep them, all I want to review is that they do not contain important information (e.g. suggesting a hack attack, software or hardware fault) and then get rid of them.
• There is no keyboard shortcut for "delete" either
• GMail diverts my browsers /-shortcut for in-page searching to in-mailbox searching

I used gmails feedback function before, but didn't get any reply.