And people who are that sensitive to their karma score are unlikely to be comfortable in LW anyway.

The thing is, a downvote campaign isn't just a losing a point or two, it's more like losing 30 points for no apparent reason in a day. Or, as happened to me for a while, having a downvote appear just about as soon as I posted much of anything. I like LW pretty much, but that really did take away some of the sparkle for me.

A downvote carries much less information than a comment. It's possible to think about whether a comment is reasonable.