i am a dinosaur and quite "uncool" when it comes to AR builds, i like to keep them simple without all the **** people bolt onto them, in general though.....
1> flat top, mono rails made out of titanium if possible instead of aluminum or you will get to be pissed one day when one of the notches break off when you put a steel rail accessory onto it
2> trijicon type holo sight
3> i actually do not like a vertical foregrip like some do, they always get in the way and snag on ****. i prefer a regular straight grip.
4> the one KEY thing i do with every build i have ever made or helped friends make is with the barrel.
a> chrome lined, more accurate and the barrel lasts 3x as long
b> has the step in the barrel where you attach the 203 grenade launcher, does not matter if you are going to register a DD and get a grenade launcher or not, there are lots of things on the accessory market that use that step now and prob more in the future,
C> TAKE THE TIME TO FIND A BARREL WITH A SLOW TWIST RATE, and match it with a lighter grain bullet to maximize its effect. this is the single biggest thing you can do with any ar15 to make yours better than 95% of everyone elses. the gun was originally designed with a slow barrel twist rate which stabilized the bullet enough for accuracy but when hitting a target will cause the bullet to yaw out of control. the wound characteristics of a 1/12 or 1/14 twist barrel sees the bullet hit and crank out at a sideways/diagonal trajectory through the body with the bullet itself almost immediately fragmenting.
the overwhelming majority of barrels you find now days will be between 1/7 and 1/10 twist because thats more accurate for target shooting, however if you are building the gun for defense (or hunting javelina for that matter =P) you want max lethality, be slightly less accurate and get a 1/14 twist on your barrel and you will get results on par with softpoints with regular fmj ammo
edit: my very first gun was a 1911 i built when i was like 12 - 13 or so with an auto ordnance parts kit and one of those L shaped screwdriver/punch combos

, family got me a stripped essex arms frame and i did the rest.
so i really like a classic 1911, for carry pieces however we use glocks, really hard to beat your standard glock for that, in fact my mousepad is one from the glock factory

. i typically have a 17 and wife a 19 , my hands are to big for the sub compacts and i like the better controllability from the fullsize deals, wear the right clothes and can conceal them just fine.