Is atheism a religion?
Is atheism a religion? is a question atheists are often asked, although it's sometimes not asked, but rather stated, "atheism is a religion." I understand the confusion because there are several points of commonality between the two: many atheists "religiously" hold gatherings where they have "communion" with each other and discuss belief in gods, souls, and the supernatural and "evangelize" their position and try to "convert" people over to it. To many people, this behavior is very similar to the behavior of religious people, so they conclude that atheism is a religion. However, while there are surface similarities, atheism is dramatically different from a religion.
Religions are opt-in
Excluding the more violent believers, most religious people accept that a person must actively chose to follow a religion in order to be religious. Atheism, at least in its implicit and weak forms, commonly described as a "lack of belief," is the default position regarding belief in gods. To put it more pithy, atheism is a religion like not collecting stamps is a hobby.
Generalizing "religion" too much makes the word meaningless
As you generalize the definition of a religion to include anything where people gather to talk about a shared interest, the word starts it become meaningless. For example, if "religion" just means people gathering together, then fantasy football leagues and scrap-booking groups are also religions. For the word to have useful definition, it should include those groups that are traditionally seen as religious.
Religions believe in the supernatural and hold rituals
While all religions are different, they do share two common traits, and that is a belief in some form of supernatural phenomena (gods, souls, spirits, etc.) and an adherence to ritual (prayers, rites, tenants, etc.). Atheism is merely the belief a lack of a belief in gods; there are no supernatural beliefs or rituals.
Some religious people are atheists
To add to the confusion, some religions, like certain forms of Buddhism, do not believe in gods. Therefore, those Buddhists are technically atheists even though they believe in souls, pray, and religiously perform rituals. Of course, religious people rarely identify as an atheist, because they equate atheism with non-religious even when it technically isn't.
You can debate a belief without believing in it
This should go without saying, but a common argument in favor of atheism being a religion is that atheists often talk about belief in gods. Of course, just like Conservative pundits can criticize liberal politics without being a Liberal, so too can an atheist criticize theism without being a theist.