"One important evolutionary step in forums like these is to hide comment scores to prevent pile-ons and "me too" type comments. "
Yup. A big problem that needs more research to deal with.
" On the other hand, have any sites tried it?"
I used to see the problem you just described happen on Facebook when people look to see who's backing who and what. It starts getting personal than simply being about the content. Interesting you brought up Twitter as a multi-vote site. It's so high-noise and full of herd behavior that we're on Laarc instead. ;)