Personally, I'd go the mefi route and charge $5. That seems to work there, and (since this site is primarily about YC's aims and its potential founders) I'm sure serious entrepreneurs wouldn't think even once about paying.
Look at it this way: people will pay for the privilege of an intelligent conversation, but trolls feel so entitled that 99% of them will never pay.
It felt funny how different my reaction was to the idea of paying at Metafilter vs news.yc. I scoffed there, but I'd pay $5 here in a heartbeat. If that's all we have to do it seems a small price to pay.
Maybe if it added up to enough there'd even be a YC company funded by it :-)
Oh, man -- your post made my math-addicted brain start estimating the financial cost of the time I spend here on news.YC. Now I need a drink, and it's only noon.
Look at it this way: people will pay for the privilege of an intelligent conversation, but trolls feel so entitled that 99% of them will never pay.