Dan's on the money.
Buffer tube: diameter, civ vs. mil
Buffer and recoil spring: length, carbine vs rifle
Hammer/trigger pins: diameter, "Early" Colt vs. rest of the universe
That's it.
Now, there are some other considerations if you start playing around with nonstandard parts. For example, an upper receiver that is sold/described as "reinforced" may not have sufficient clearance for any bolt catch other than the standard one (ruling out accessories like a "tactical bolt catch" or the Magpul BAD lever). But these are special cases.
The whole .308 thing is a giant can of worms. I personally stick with the KAC (and DPMS) "standard" because KAC charges way too much money to ever go out of business.
