At the rate I (slowly) accumulate the imaginary internet points here, it will be the better part of a decade before I end up with the ability to "endorse" any other comments. I don't really care about that, except you're locking out those of us that don't have a following here, or name recognition. We, collectively, have seen how "voting" works -- here and elsewhere -- after a very short time, people just upvote based on who the person is, not the comment. Endorsements will work the same way.
Being limited to only one "pending" comment at a time, and the very high probability that I'll see significant delays between being able to comment, pretty much guarantees that I'll leave and find my tech news elsewhere.
Perhaps that's fine with you. But it's a shame to ruin a community to solve a problem that, quite frankly, doesn't exist.