Power gives slightly more of a boost to damage than main stat does. This is balanced by main stat increasing your crit rating slightly. However, once you're at your crit goal, or if crit isn't useful to your spec (which it isn't for some bc of procs that auto-crit) you want power over main stat.
Umm, yes. Almost every spec except Focus/Rage is better served by main stat, and even Focus/Rage isn't by much.
Power gives 0.23 bonus damage, main stat gives 0.21 (it's 0.20 but when you factor in the 5% buff everyone has at all times to their main stat, you end up gaining 0.21). Even more than that if your spec has a percent buff to main stat. Factor in the bonus to crit and main stat wins. Having a crit% goal is dumb - that isn't how diminishing returns work (they're on the crit rating not the crit percent).