Larger forums like HN and Reddit have the economies of scale on their side. They have a lot more time and money to throw on solving the problem than a blogger or someone who runs a phpBB-forum as a hobby.

Traffic + downvotes. A blog may have downvotes but not the human traffic to utilize them

HN have a different approach. Here in HN, new accounts are restricted to comments only, the restriction will slowly remove as the account get older and have amble "karma" (sorry, I am not sure what term for here in HN) then they will gain the ability to vote later on.

Also HN prevents people from voting on parent comments if they respond to it. So if they commented it, then it is considered as voted. If they up/downvote the comment and then posted a response to that, HN automatically replace the vote to comment response.

I like this approach because it is a great way to have engaging discussion than trying to drive meaningless and irrelevant comments as top comments.

HN has @dang!