Not just Ask HN questions but people will think thrice before commenting on anything and everything. The quality of comments on HN is pretty good as it is. I think this is a bad idea. Sometimes not showing something can be a good thing.
Just take a look on some articles on searchyc.com.
I'm not saying that the quality today is any worse than the quality a year ago (it's very difficult to judge that metric), just the submissions and comments are different, which is to be expected of a growing community.
I'd say that is a good thing. Since coming here I have started to carefully consider posting each and every time.
I like to think my post quality has gone up because of it!
(also, the majority of commentors are not on the leader board so this effects only a minority)