The Guardian could really do with a downvote button or similar. It seems one of the easiest ways to deal with iffy comments. Not sure why they don't have one.
I think there are disadvantages to a downvote button in some cases, the current setup allows for every comment to be seen as an equal (as long as it passes the community guidelines), which can be valuable when discussing particularly contentious issues. I guess it's always a choice as to what community you are looking to foster.
(Disclaimer: I'm a dev at the Guardian, but not in an area related to commenting)
Guess there are pros and cons and it's a bit of a personal preference. I was thinking that the Hacker News comments system works pretty well. Maybe they could clone that?