I think the main reason bitcoins will become mainstream is because it is anonymous and you do not have to pay currency conversion fees like the other payment processors you mentioned.
Bitcoins aren't anonymous, they're pseudonymous.
pseu·don·y·mous/so͞oˈdänəməs/
adjective
writing or written under a false name.
Read more about it here:
http://bitcoinsimplified.org/learn-more/anonymity/4chan is anonymous, because there are no usernames and no way to tell who posted what/follow anyone.
Bitcointalk is pseudonymous. I don't exactly need ID to sign up, but people can see everything I do and link it back to one single person.