The only thing I hate is that if you have multiple accounts on the iPhone, you have to drill down into each account, then back out, in order to check the other accounts.
And I HATE the fact that if, say, a mail server is down or something when you're trying to send a message, it won't give the option to use another account, it just sits there spinning and sticks the message in the outbox. It also doesn't provide any obvious notification that your message wasn't sent.
I'd also like to be able to change SMTP servers in the mail window like you can in OS X mail. This would help alleviate the above problem in some cases, but it's probably a little too much to expect.
And signatures? WTF can I only have one? It's not that big a deal to have a different signature for each account, is it?
Also the interface is clunky and garish.
But this is really just a rant. All I can hope for is that Apple takes mail a little more seriously in the next iPhone OS.