I wish google would just implement a way to add a list of blocked domains to your search preferences. Then I'd never have to see crap from mahalo, expertsexchange, or the like ever again.
A way to opt-in to using a community managed list of bad domains would be even better.
When they were playing around with their SearchWiki system, there was an 'X' next to all results that you could click to make them go away, which I was pretty excited about, until I realized it didn't actually permanently banish that site; just removed it from the current results page.
There are a bunch of greasemonkey scripts to let you blacklist specific sites from Google results, but relatively frequent changes to Google's results page seem to make them always fall behind, so it's hard to find a consistently working one. This is the best-maintained one I can find, and it's actually not currently working: http://userscripts.org/scripts/show/44418
Didn't google some time ago have these up and down arrows, like here on HN with comments, which I never quite learned as to what they were for. That is not quite far off from marking sites as spam and I used the up and down arrows only perhaps once in the entire time.
Search is different from sites like this. Here we know we are going to spend some time reading interesting content, but not quite what we will be reading. When searching you know you needs some information and what you want to do is find it, preferably instantly, and get out of Google immediately. It is easier to click on the next link than mark some site for spam or click the up and down arrow.
A way to opt-in to using a community managed list of bad domains would be even better.