I use Gmail a lot now. I also have come to know and love a few keyboard shortcuts. I can't imagine dealing with a large volume of email without them.
You have to turn them on under the mail settings first.
u will refresh the current mail search or your inbox if you're looking at your inbox.
u will return you to your mail search, label or inbox if you're looking at an email.
j/k to navigate between emails in your inbox view.
x selects an email, and dumps you back at the message list if you were in an email.
s applies a star to selected emails, or stars the email you're reading.
+/- will flag/unflag an email as important (if you're using priority inbox)
e archives selected emails, or archive an email you're reading.
r replies to an email.
m will mute selected email threads so it won't reappear in your inbox when more posts are made.
gl lets you type the name of a label you want to view.
? brings up the help.
Most important button to me is u, as it refreshes my inbox when my android phone has chimed to let me know that I've got an important email, and the gmail UI hasn't shown it yet. The androids. They always know.
(Photo from Geek Sugar)