My two biggest annoyances with Gmail:

One of my accounts has this bar at the top reminding me that a couple accounts are linked. It says this notice will remain for a few days, then disappear... it's been there now for 3 or 4 months.

Secondly, Gmail auto-trims signatures. This is annoying by itself, because I don't think Google should get to decide, by fiat, that signatures are passe. Worse, the trimming often trims much more than intended -- sometimes whole messages. It's totally unprofessional when e-mails to clients & partners go out sans signature, closing, and the bottom half of my e-mail.

Naturally, getting a hold of Google for help with this is a laughable Sisyphean endeavor.