Whatever kind of contrarian Less Wrong thinks is valuable. It's not completely specified. I'm not sure I see how tone comes in.
I'm thinking of the responses to critics of late. Even the arseholes are slightly worth listening to, but tone arguments are a way of not listening, and this may miss something important even if it's often all the response it deserves. No-one's obligated not to use it, but it's a good exercise to be able not to, particularly for the benefit of onlookers.
I'm worried that LW doesn't have enough good contrarians and skeptics, people who disagree with us or like to find fault in every idea they see, but do so in a way that is often right and can change our minds when they are. I fear that when contrarians/skeptics join us but aren't "good enough", we tend to drive them away instead of improving them.
For example, I know a couple of people who occasionally had interesting ideas that were contrary to the local LW consensus, but were (or appeared to be) too confident in their ideas, both good and bad. Both people ended up being repeatedly downvoted and left our community a few months after they arrived. This must have happened more often than I have noticed (partly evidenced by the large number of comments/posts now marked as written by [deleted], sometimes with whole threads written entirely by deleted accounts). I feel that this is a waste that we should try to prevent (or at least think about how we might). So here are some ideas: