Quora tried something like this and it absolutely destroyed the site. I was on there when basically all content was unpaid, and I think it mostly worked pretty well and you could get some high quality answers. After they went paid (the Quora partner program, I think it was called), it basically turned into Yahoo! Answers both in question and answer quality. Every time I’ve visited Quora since, it kinda has the same effect as looking at those pictures of people as they use meth for longer and longer; the site just keeps getting more and more unusable in both interface and content.

But I’m sure it’ll work out just fine for Reddit