The problem with Humanism
As an Atheist, if anyone asked me how I would define my world view, and a subsequent system of ethics, I would have to say that humanism is my chosen philosophy. It appeals to me that as humans we are responsible for our own destiny (whatever that may be). As much as I like the ideas endorsed by humanism, I do believe that it suffers from a few problems. The main problem being that of an assumption of a certain level of intelligence or at the very least a certain level of education for an individual.
As systems of ethics, religions have survived very well from an evolutionary point of view because they short cut a lot of critical thinking to arrive at (in most cases) some very important ends. I'm not talking here about the fanaticism of strapping explosives onto oneself, or even the evangelical fever of attempting to convert everyone to your way of thinking. I'm talking about basics like "Do unto others as you would have them do unto you" or "Thou shalt not murder", "thou shalt not covet" etc.... These basic ideas come through not only in many religions, but also in humanism itself. The difference is that religion takes a massive short cut by claiming "God says <insert ethical code here> and therefore you have to do it otherwise <insert divine punishment here>". This way of thinking, this threat of supernatural consequences has historically been extremely effective in keeping vast societies of human beings generally adhering to a similar set of ethics. This is one reason I believe that the various memes associated with religion have been very successful in perpetuating themselves throughout our evolutionary history. This is how it has been ever since humans developed the ability of abstract thought and consequently tasted of the tree of the knowledge of good and evil.
Humanism has no ultimate individual punishment for its adherents in this life, and no afterlife. So instead it relies on the ability of the adherent to think through not only the possible individual consequences of any particular action, but also the more ethereal social aspects of the action. This suffers from the classic prisoners' dilemma situation in which acting for individual benefit will always (from the point of view of the individual) have a better outcome than acting out of a social conscience, but acting with a social conscience will have the optimal group outcome.
I remember in my philosophy course at university, when my lecturer first explained "prisoners' dilemma" to us and discussed possible ways of solving the "prisoners' dilemma". His analogy was of a Godfather character who would say to each prisoner, "I need you guys out of jail as soon as possible, so if either of you squeals, it's concrete shoes for you". This brings about an obvious solution to the decision matrix and forces the prisoners into acting for the benefit of the group instead of for their own benefit. Religion is the ultimate Godfather with concrete shoes. There is no need to think through any possible consequences in this lifetime, and no need to trust in your fellow man to do the right thing. Any decision matrix is always going to be clouded by the threat of divine punishment aimed at drawing the individual towards whatever the pre-determined decision should be.
The tenets that form most religious ethical systems are based on a mixture of heuristically determined social values, and specific values decided upon by those in the hierarchical power system of the religion. These ethical tenets aren't necessarily bad in and of themselves. In fact, some of them become self evident when you take a humanistic approach to ethics. Others become meaningless, or even dangerous when taken out of their historical and/or geo-political setting.
The problem for humanism is that unless someone is willing to put a serious amount of thought into the way they live their lives and the value system they employ, there is no way to ensure a socially optimal set of values or ethical tenets. This amount of thought requires an understanding of the prisoners dilemma scenario (not necessarily exactly as it is stated, but certainly the ethos of the problem), and then a giant leap of trust in your fellow man (humanist or not) to make the same decision to benefit the group. Obviously this dilemma gets more and more complicated the more 'prisoners' you add to it, but the end result is the same.
For the people that profess to adhere to humanism, generally they are willing to put in the necessary thought and act accordingly for the benefit of the human species. The issue is that there appears to be only a small percentage in any given population that are able and willing to employ the necessary forethought themselves to arrive at such a set of ethics. For whatever reason (laziness, ignorance, stupidity, lack of education) the vast majority of humans still require the short circuit provided by some divine consequence, to cajole them into behaving in a way to benefit the group instead of themselves. I just can't see at this point in time that humanism is ready to replace religion as a mass means of crowd control. I would like to think that humans would eventually out-evolve the need for religion (at least for the purposes of determining a system of ethics), but evolution has no specific direction or higher purpose at all, so unless the inherent inefficiencies in religion out-weigh the 'benefits' that religion provides, this won't happen.