There are many misconceptions about Atheism. Some are larger than others. However, as pointed out in a previous post, the main thing that people get wrong about Atheism is the definition, what Atheism IS. You'll get all kind of accusations about what an Atheist believes, who we follow, even claims that we have a doctrine that all Atheists must follow. However, what this all boils down to is the biggest misconception of them all.
"Isn't Atheism a Religion?"
Short answer, no. But in order to get the reason and to understand it, we must first focus on what a religion is, and why it cannot be applied to Atheism.
A cause, principle, or system of beliefs held to with ardor and faith.
A specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects.
The belief in and worship of a god or gods, or any such system of belief and worship, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
The basic definition of religion is a system of beliefs held by a group of people, usually involving worship or rituals of some kind. So why does this not apply to Atheism? Well, it really is quite simple. Atheism doesn't involve rituals, there is nothing to worship, and there is no set system of beliefs. Some people like to fight that last point. The usual claim is that Atheists believe there are no gods. As stated in the other post, though, this is incorrect.
Atheists have a lack of belief, not a belief in anything. This is all Atheism is. Apart from that, an Atheist can be whatever (s)he wants, even take up certain religions like Buddhism. With all of this in mind, it is very hard to see any reason why anyone would assume Atheism is a religion.
If you have any more questions about this topic, you can check out this link on About.com.