For another thing you might or might not have overlooked: doesn't a person endorse a comment for the same reasons she would upvote it? If so, upvoting would endorse it, and you don't need a dedicated 'endorse' button. The pending comment can stand out with a [pending] tag or a different shade. That would make endorsing weave itself more naturally into users' current habits. On the plus side, if a 1k+ karma user is into the habit of upvoting fluff comments it will soon bite them when they do that mindlessly to a pending comment. Pavlov is a good teacher.