Most of the reason is that having to sign in with a Facebook account establishes a real identity and hit-and-run snipes and insults are harder to do when one can't maintain anonymity. So, like TechCrunch said, its trolls have dwindled and its comments are decidedly gushing -- but they prefer that over the sheer "anonymous" nastiness that showed up on a daily basis.
In journalism, we know the importance of using someone's name and spend a lot of time correctly identifying that person in a story. A commenter saying something rude, crude or malicious is easy when it's done anonymously, but requiring a real name and account to comment and that element tends to disappear. Does that mean it's neutering our free speech? Not really, but it is creating more accountability in a world that judges people for what they say and who they are online.