So mefi is the same as HN but you even have to pay on top of it? ;P
The advantage of one, small, upfront payment is that it discourages sockpuppeting, and thus anonymous sniping. The only way to 'downvote' something is with another piece of text with a username on it ... a lot more revealing of motive than a reflexive button-click. Discouraging for sick puppies.
You know (or don't) ... like The Well was back in the day.