Reddit's not exactly having trouble attracting a mainstream audience. It attracts a far more "normal" crowd (as in, broad and varied) than any other social news site, with the possible exception of stumbleupon and twitter, which are completely different kinds of sites and not conducive to AMAs anyway.
Also, the (apparent) lack of ability for follow-up questions and conversations based on answers seems to be a big downside. Be it reddit, HN, or Wikipedia, nested conversations are a no-brainer in 2011.
To be fair, I've often "read" reddit IAMA's by just looking for the original submitter's answers; the followup discussion is not necessarily equally valuable.
