All my AR's are equipped by what I employ a given rifle for, but here are a few basics I'd say are key:

Good iron sights (Back up or standard)
Good optic
Quality sling (I prefer 2-point)
White light (Pressure pad and switch/cap activated)

I think those are some good basics to have on the rifle, and every one of my setups has at least those. You can certainly go to town on a rifle, but I'm of the mindset that I want everything on the gun to be functional, and keep it as minimal weight as possible. If I don't use it or need it, it's coming off the gun. I do like certain stocks (sopmod style w/ storage and cheek rests), vertical foregrip, and things like that, but it's all preference.

Here's the specs on my go-to AR, just for ideas and reference - this one is certainly equipped to the hilt, but gives some ideas of the possibilities:

LMT MRP
Larue APEG grip
Tango down QD stubby VFG
GG&G rear iron sight, Troy industries M4 front sight (both BUIS)
Modified, stippled Larue index clips
LMT SOPMOD stock
LMT 2 stage trigger (swapped out the LMT mil-spec)

Optics & Accessories:
Aimpoint T2
Spuhr HRT mount
Steiner DBAL i2 (9007 all IR model)
Surefire scout (white & IR)
Surefire SFMB Muzzle Brake
Surefire SOCOM 300 SPS