a good compensator i would say is a must
It depends on the purpose, IMO. For a competition gun, when trying to squeeze out slightly faster splits and slightly faster stages at all costs, a compensator can be very helpful, yes. A whole lot of 3-gun and top USPSA competitors use them, for that reason.
For a gun that is going to serve in a defensive role, though, I think most of us are better served with a flash suppressor. A compensator makes a 16" .223 several times louder, and having a "flash and blast amplifier" on the end of your barrel while firing in low light without hearing protection (especially in an enclosed area, like the hallway in your house) is going to slow down your followup shots rather than speed them up, and isn't going to do good things to your situational awareness either. I personally like the Smith Vortex, but a lot of people I respect like the AAC Blackout, and the basic A2 flash suppressor isn't bad either.
As to the broader hardware question, the number one thing in my mind is to
make sure the carbine is assembled correctly (gas key screws properly torqued and staked, castle nut staked, buffer weight and spring type are appropriate for the gas system length and port diameter) and that good magazines are used.
Beyond that, appropriate accessories depend heavily on the role the rifle needs to fill.
My needs are basically a 0-300 yard carbine for USPSA type shooting and defensive standby, and for those purposes I see a light, an optic, and a Vickers-style 2-point sling to be the three most useful accessories. The light is the only one of the three that I would see as being pretty much a necessity; the other two are optional but helpful.
For a long-range target gun or varmint gun, obviously a much different set of accessories would be appropriate.