I've looked over the page for keyboard shortcuts and I've noticed there's not much of anything to allow for more convenient upvoting or downvoting of posts. There are designated modes for "vote up" or "vote down" while viewing the post listing, but there doesn't seem to be a designated shortcut to switch between modes.
So long as the mode selection dropdown menu keeps focus you can swap between modes using WASD or the arrow keys, but clicking a post shifts the focus away. This means the only highly efficient way of voting for posts that exists at the moment is restricted to either purely voting up or purely voting down.
This is still useful at times, but it seems to me that it would be much more useful if expanded upon. The "score" associated with each post is a useful measure by which to order the posts in a tag search and it's used to generate the "Hot" listing ({order:rank}). Making voting more convenient might encourage more active participation. I conjecture that most positive votes that posts receive comes from the automatic vote applied when users add it to their favorites. It is my opinion that increasing voter participation independent of favorites would make the "score" value corresponding to a post more meaningful than it currently is.

The first potential option for implementing shortcuts for voting would be to add a designated keyboard shortcut to shift your current mode to a mode listed adjacent to it (forward and backward) while viewing a post listing. The 'k' and 'l' keys might be good options for this.

The second potential option would be to add a shortcut while viewing a post itself to vote up and vote down. Maybe a shortcut to remove any vote, maybe hitting the shortcut corresponding to one's current vote removes it, or maybe un-voting would be left manual.

A third option might be to implement it in a manner similar to the way that the "tag script" mode is implemented.

I personally like the option that I listed first. People who use browser extensions like Imagus or userscripts like some of the ones listed in about:userscripts can view posted images in full size without visiting the posts themselves. Having a shortcut to switch between modes might also have additional applications outside of this context.

The main problem is that the best keys are already taken. My preferred choice is 'v' for upvote and 'shift+v' for downvote. Easy to type and easy to remember. Unfortunately, 'v' is already used for toggling posts between samples and full size. I'm thinking of moving the existing 'v' shortcut to 'r' (for 'resize') so we can have 'v' for upvote. This would be a painful change for people used to the existing shortcuts, but there aren't many other good choices and I think voting is important enough that it should have an easy and memorable shortcut.