Multiple small parts all of which can break. Extractor (w/spring & pin), ejector(w/spring and pin), gas key screws can loosen, firing pin can break, cam pin(not usually a problem), bolt itself can break, gas rings can become dislodged or break. Carrier can become clogged with carbon.
A bad gas tube can damage the gas keyway.
Improperly staked keways can come loose or break.
Firing pins can bend.
The retainer pins can break over time.
The extractor spring can wear out.
The extractor can wear or break, latter of the two least likely.
The gas rings can wear out or become out of stagger.
The BC itself can be damaged or scarred which may impede function.
Round overrides can cause a lot of problems and damage.
Charging/chambering the AR by simply pulling the charging handle and letting go can create big issues and round overrides. You should lock the bolt to the rear and use the bolt release. (know this all too well first-hand from my last deployment thanks to an idiot E-7)
Did I miss anything fellas? That's pretty much the learning curve for combat loud-out and combat environment cleaning issues we looked for.