This explanation may be too simple; if so, I apologize.
I assume you are speaking of AR-based "service rifle" competition, as opposed to "match rifle" competition. Broadly, a "service rifle" has to appear (emphasize appear) exactly like a US service rifle (there is one exception for Post-Ban rifles with no flash suppressors). Several armorers - Accuracy Speaks, Fulton Armory, Compass Lake Engineering - have web sites, and that will help you understand modifications. Also, the Bushmaster on-line catalog carries some kind of CMP / DCM weapon, so you can look to see what features it has.
My $.02, but the most common mod's (in order) probably are: trigger job; improved sights; steel tube around barrel; match barrel.
Again, broadly speaking, match rifle is much less restricted. "Space guns" are not uncommon.