What would be high on your list, out of interest?
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.
Anythin...
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.