For me personally, writing email faster. It's really easy for me to get immersed trying to write the perfect email or forum post and burn through 40-60min without noticing. They're not even necessarily long, just excessively pruned and reformatted. Getting comfortable with an email with all the important content and okay phrasing saves me a bunch of time.
On second place, priority filtering, i.e. separating email to respond to from subscriptions, offers and notifications. Category filters are nice, but I don't think they're making me more productive.
Anything beyond that is kind of marginal for me. Shortcuts for inserting phrases are fun, but I don't use them that often. If I had to deal with repetitive inquiries, they'd probably be more important.
I'd add using keyboard shortcuts, which are available in desktop clients and GMail.
How frequently do you think you *should* check email? You can also say how frequently you do in comments.
Personally I'm sold on thinking you should check it around once a day, not necessarily without fail. That increases focus on both email and non-email, and minimises getting sucked into distractions. But some people I know disagree. Some believe in getting notifications whenever a new email comes in.
For anyone who'd like to check email less often and uses GMail, I recommend using http://inboxpause.com/ and this full screen compose link: https://mail.google.com/mail/u/0/?ui=2&view=cm&fs=1&tf=1&shva=1
Edited to add: I'd recommend everyone at least try checking only once a day, at least for a few days, to see if you find it more productive and/or relaxing. That'd be a big enough win to make experimenting worthwhile.