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.
I primarily follow HN for news, with the option of discussion and clarification of said news. Many (most?) of my comments are buried deep in the comment tree, as responses to specific comments, and tend to get one or two responses.
And that sort of thing - responses back and forth, clarification and additional questions - are the sort of thing that won't be able to be done. There is no sense in posting a reply to someone with less than a thousand karma if it's buried deep in the comment tree, even if it's something the parent commenter is interested in, even if it is something that other people would be likely to want to have answered. Because it is a niche conversation, and there are so few people visiting it that it is unlikely that someone with 1000 karma will visit it.
If I cannot comment, why stay? Most, if not all, of the interesting articles are crossposted elsewhere, places where I can comment.