Maybe Wikipedia wouldn't work unless you made the whole service "for-pay" in some way. The nice aspect of requiring users to pony up some amount, which then becomes a pool that you make micro-payments from. So you get the gating aspect of a for-pay service combined with the market forces resulting from the micropayments. And yet, the users are really not paying the $5 up front.

Wikipedia might well not work, simply because the free one works so well.