Setting the threshold at 2 seems a bit high to me. It basically requires that (on average) at least 1 person upvotes every comment you make. That's unlikely to happen in a nested thread of back-and-forth conversation. And it's less likely to happen for people that are a little late to the conversation but still have some valid input. I tend to fall into the latter category -- I'm often reading posts that are several days old in my RSS reader.

Setting the threshold around 1.5 might work better. Or perhaps whatever threshold might allow 75% or 80% of readers/contributors to vote.