In reply to NottsRich: Good to see a discussion on this subject. Some good comments above and I'd second Bruce Tremper book recommendation. You seem to be after some principles so here's my take.
First - ice.solo made this comment "plus, its one thing to predict avalanches, and other to do something about the risks." There's an incredibly important but quite subtle point behind this . There's a natural assumption that if we are aware of a hazard and it's potential consequences we'll avoid it. That seems quite logical but, disturbingly, it's not necessarily true. Studies of education-based safety programmes in relation to sexual health, drug use, driving habits and avalanches show similar findings. The key point is that being aware of the risks, does not, in itself, make you any safer. *Changing your behaviour* is what makes you safer. There's an interesting paper on this subject here
http://www.sunrockice.com/docs/Sex%20&%20drugs%20IM%202004.pdf In short, understanding the human decision-making side of avalanche safety is critical.
Back to snow. An avalanche starts when the gravitational load at a point in the snowpack exceeds its strength. This can occur because 1. The load increased or 2. the strength decreased (or a bit of both.)
Things that increase the load on the snowpack include humans, more snow (note that snow deposition rates from wind transport can be WAY higher than from falling snow) & rain.
Things that reduce the strength of the snowpack are things that weaken the bonds between ice crystals in the snow. Examples are anything that causes rapid warming (e.g. rain, sun) or the slower process of facetting.
You mentioned failure mechanism. It might help to think about "right way up" & "wrong way up" snowpacks. A strong layer (e.g. windslab) over a weaker one is the wrong way up - it is more likely to slide. The sliding layer can either be a weak layer (i.e. poorly bonded crystals) or a good sliding surface (e.g. an ice layer.)
One other thing - warming then freezing. The *overall* tendency of melt-freeze cycles is to stabilise the snowpack. However the melting part of the cycle is weakening it, the freezing is strengthening it. A slope that's bullet-proof cramponing first thing on a spring morning and that would not avalanche with the entire UKC community dancing on it changes into a weak, slushy unbonded mess that could slide all the way to the ground in the afternoon sunshine. When it comes to melt-freeze cycles, timing your run is critical.
There's any amount of information in the books about terrain, the three (or five) "As", weather and it's effects and how the weather and terrain interact to make the snowpack how it is. But good on you for asking the questions and getting us all thinking.