What are the most common worn/broken parts in the AR system? All I can think of is:

Buffer Spring
Extracter Spring
Extracter
Firing Pin Retaining Pin
Firing Pin
Bolt

Please tell me if I missed any...

knock on wood




but i have a few rifles that are near the 10k mark and have never needed anything changed

I keep some  carrier rebuild kits,spare bolts,disconectors,bolt catch mag catch assemblys,hammer,trigger.I have multiple ARs so I keep more than one of the same item around.I have 12 and 13 year old Bushmasters whose only parts changed were barrels,stocks and the carrier when the closed bottom ones started to appear.So I ran the same carrier and lower parts and those parts are still kicking after many thousands of rounds as the rifle changed configurations a few times.

I've personally seen two broken bolt catches.