I thought about this a while back - a big negative to it would be that if someone sees traffic coming from HN and comes to sign up and answer questions and feedback in the comments. I've seen lots of great first comments after signing up by people who had their content reached.

A similar idea would a hard limit on comments for new people for the first day? 3 or 5 perhaps?

We do a 5 day comment ban at http://forum.doom9.org/ works very well. Spam is a rarity, and the talk is quite good. Example: http://forum.doom9.org/showthread.php?t=145561

Perhaps, but that might affect lurkers (perhaps even long term lurkers) who finally decide they have something useful to say and want to say it - now.

I tend to mostly lurk, but I did find myself in that situation a couple weeks ago.

Doom9, another forum I visit, does this for 5 days to force new users to read the FAQs and guides before asking their question.

It works well, IMO.