An AR from one of the main manufacturers (Colt, Bushmaster, Armalite) is going to be pretty well made, so think carefully before accessorizing. The existing mil-spec coating system has been proven to work properly for thousands of rounds; don't break what's already working. Likewise, the Ti firing pin might reduce the likelihood of a slam fire, but on the other hand it probably won't last long anyways (the aftermarket Ti strikers for Glocks are reputed to be rather short-lived).
As with all production rifles, think trigger, sights, reliability, and fit.
Either have a trigger job done by a dedicated AR smith, or go with one of the drop-in setups. Either way, be darn careful that both sears work correctly, and that you get a good consistent break.
Fix the sights to your liking, including optics (heck, go wild and put a Trijicon tritium front sight on).
For reliability, check the feed ramps cut into the barrel extension (may need to be polished or opened up). Make sure the chamber is smooth and cut to the right dimensions. Disassemble the bolt and make sure there aren't any burrs on the extractor hook or ejector. You might want to use a fine stone to slightly break the edges on the extractor hook, to prevent brass from getting shaved off the case rim and bollixing up the action. Lastly, consider getting one of those polyurethane extractor spring helpers.
Find the pistol grip that you like best, and buy it regardless of price. Likewise, try the old triangular as well as the new round foregrips.
If you're just itching to put new hardware on it, get some of the green followers and update your mags. Especially for the AR, the mags are the heart of the rifle, and deserve much of your attention.
