In response to the questions:
1)The firing pin in an AR weighs less than an M14/M1A pin so it gives the primer a lighter tap. Also, the factory primer on 5.56 ammo has a thicker cup material then usual. If you handload, stick with Remington 7 1/2 primers and you're fine. Use any other small rifle primer, and it's anybody's guess.
2)There is no hard and fast answer to barrel life. Whether the barrel is chrome lined, the rate of fire, type of powder (specifically ball vs extruded), barrel steel, all play a role. The barrel is good until you decide it doesn't meet your needs any more. One thing to remember; seeing some erosion at the throat ain't necessarily time to junk the barrel. Erosion effects velocity, but usually doesn't impact basic accuracy for quite a while.
3)Main wear areas are bolt rings, which are intended to wear out eventually, after many thousands of rounds, and that's about all that comes to mind. Other parts do break from time to time, but assuming a properly built piece, any breakage will tend to be random. I have heard of bolt heads cracking at the hole for the pin, and ocasional extractor breakage. I keep a spare firing pin, extractor, and extractor spring for all my ARs. The only replacement I've ever had to do was the extractor spring on the SP1 carbine, that got tired after 4-5000 rounds. I've put Wolfe heavy duty extractor springs in all 4 of my ARs, have one additional spring for each of them and never expect to need any more[:D]