The idea of Atheism being anything more than the disbelieve in god(s) is something which is stopping both Theists accepting it and Atheists from being condescending.
Whenever I have met criticism about my Atheism it has always been made in the way of sweeping generalisations. People seem to assume that because I am Atheist I have to subscribe to the “Big Bang” theory or in the “theory of Evolution” which, although I do, must be very annoying for those Atheists who don’t.
Equally, the idea that Atheism has to be free from religion is another idea which is getting in the way of Atheism being truly accepted. Several mainstream religions do not have god(s) in their believe system and so someone who is a follower of these would be an Atheist as well as a whatever-they-are-first-ist.
Basically – Atheism is not a common believe structure and shouldn’t be treated as such, and the fact that a lot of people want it to be treated as such is pathetic and backwards.
You are correct in much of what you say, but on this site I am discussing my own personal flavor of atheism. Also, I think that the atheists who are being condescending are doing so not because of misinformation about the definition of atheism but because of arrogance, intolerance, or other personal problems. They need to be shown the error of their ways just as much as theists who have preconceived notions of what atheism is do.