The multiple redirects upon login are alarming, at the very least.
Login often (all the time?) hangs upon logging in twice in one browser session.
A refresh usually remedies this. Still a pain.
Need a way to display messages by label differently. e.g. Show all messages that are not a certain label.
There are no headers for the columns on the main interface.

(Click image larger view.)
I mean, I know Google is known for their simple interface, but there are some things an interface just needs. Column headers (no matter how obvious it is what type of data are in the columns) are one of these things.
What’s up with the weird code that runs the pages?
There almost nary an XHTML tag in the stuff - all JavaScript. Not only that (who really cares what the code looks like), which among other things makes right-clicking links in order to open them in new window/tabs, print, etc. impossible, since the browser doesn’t recognize them as links per se and only gives you the generic options as if you’d clicked in space.
That’s downright rude, if you ask me.
Slick use of layers and javascript to enable fast access to (and from) things like Search Options, Reply/Forward fields, Create a Filter, and especially the other messages in the same “conversation” (as Google calls a thread of emails).
Props on the accessibility front for offering keyboard shortcuts.
When viewing a conversation, there is no indication of which messages have been read, which ones haven’t.

(Click image larger view.)
There is no way to save a message while you are composing it.
p.s. No, I do not have any invites to hand out. I was only cool enough to land my own invite until after they stopped giving those out. Sorry. You’re first on my list if I get one, though. I promise.