The following make Gmail almost the best experience possible, so any complaints I have should take into account that for over 7 years I've, for free, been extremely satisfied with it. Still some annoyances, though.
two newlines at the top of each reply
Undo send it a god...send. I just wish I could customize how long I have to undo.
Gmail 2.0 for iOS
Amazing, amazing update. Multiple accounts plus I like how it looks. I'm not snobby enough to care which font they use.
http://iamnotaprogrammer.com/One-reason-gmail-ios-still-sucks.html documents the problem people have with HTML emails (no tap to zoom, spacing is sometimes wrong), though I don't think that one reason makes the whole piece of software "suck"
tapping a Notification in Notification Center goes directly to the email in question (good!) but it clears out the notifications from Notification Center (bad, because that should be up to me, especially with multiple emails.
Evidently you can only add 5 accounts. This doesn't affect me (all my sole proprietorship emails forward to one place, and I only need a second full account)
To remove an account, you have to sign out of all accounts.
That almost affected me, when mail for my second account wouldn't refresh and decided to remove the account and re-add it. Luckily I was patient and it started working. It would have been annoying because I have an app-specific password for my personal account (thanks to 2-step authentication) and would have had to generate another password for it).