Where necessary, implement some form of community moderation, much like here, unpopular comments are greyed out, on other sites, like Reddit, they are hidden until requested.
Forced anonymity will forever be a pipe-dream anyway I think.
It's a tricky subject, I agree, and it's clear we lie on different sides of what we consider to be acceptable, but ultimately, it's quite hard to have a community drive site the doesn't devolve -- HN, Reddit, Digg, and every other online community is a testament to that.
The fact that it's technically non-trivial to identify a person absolutely makes it harder to keep trolls away, as they're always free to get a new IP address and re-register as another username, which makes it harder to enforce standards in the community.