AA is often described as religious by those who are not recovering alcoholics and who have their own axes to grind. I've even seen a sociological analysis that "proves" AA is a cult - it has it's own vocabulary, "sacred" literature, and it "venerates" Bill W and Dr. Bob. Well, on that basis, a bunch of mechanics reading a Haynes manual, or a set of Formula 1 fans, could be said to be a cult. I believe that that particular analysis was flawed, because there's usually someone either making a lot of money, or having a lot of sex, or both, leading a cult, and I don't see either of those happening in AA.
As far as being religious is concerned, it's a question of semantics and pedantry. Sure, the word "God" is mentioned throughout the Big Book. Sure, some meetings end with the Lord's Prayer. However, "God" is always qualified with the words "as you understand it". This allows people of all faiths and of none to take part.
And when you dig a little deeper into AA's moral philosophy, you'll see that it is the behaviour that lies at the root of all major religions: honesty, acknowledging one's failings, seeking to improve, having a mentor, being self-aware. Indeed, so widespread are these behaviours in spiritual practise, that it could be said that they predate religion, or at least those socio-political power structures that are called religions. These practises sit with the others that overlap the major religions - charity, cooperation, caring for others. (This is before the St. Pauls, the Shias and the Sunnis etc created their misanthropic factionalist perversions of their religions.)
If this is so, then the very behaviour that is characterised as being "religious" in AA, and more often "Christian", is in fact a non-exclusive, universal, expression of spirituality. It is what enables us to become better human beings, without having to take sides in a manufactured argument. It gives us a sense of perspective about ourselves and the world around us, and a sense of gratitude for the life we lead.
It's because of this universal wisdom that AA has outgrown its mid-West Protestant roots and is now embedded in cultures and societies all over the world. And that is the greatest proof that it is not a religion.