I agree that the quick 5 second sign-up is the downfall of most websites (Ironically WordPress is that way ), I would not charge people to use my website either… I think your answer is in your question “Why do websites with an easy signup fail” because they have easy sign up.

I am a believer that the form should be long, then there should be a small waiting period (which is how I run my clans/alliances in games) if you forget all about me in 48 hours… or you just signed up to write one reply and never will come back to even see the follow up conversation… then why do we need you here? So just making a larger requirement for signing up would be a much more efficient system then requiring payment.