I am probably too young to think of one with the same restriction. It doesn't work because there is no real gain.

The point seems to be to improve the quality of comments.

Look. The intention is good. But I don't like this feature. Since most comments will be endorsed eventually, we won't gain much from this restriction.

Some people don't understand sarcasm (I am with this group!). Sarcasm is not necessarily a bad thing and can be quite fun to read. This is not a forum where people post scholarly comments. As long as the comment is genuine I personally consider it a good comment.

Now some people have a more compelling reason to "karma-whore".