Looks like someone's rockin' the Magpul accessories. Looks niiiiice!
Here's a suggestion...if you get the chance, order and install the Rock River Arms 2 stage match trigger. It's too sweet for words.
You might also take a look at Stark pistol grips for the AR. I just did a lower build myself; the Stark gives a really nice feel to the lower; and that trigger is to die for. Unfortunately, the upper it is mated to--a stock Armalite full length--isn't the most accurate in the world.
My next step will probably be to install a better barrel, with a low profile gas block.
EDIT: Hey, just noticed. You might have beat me to it, but make sure that your castle nut (holding the receiver extension/buffer tube) is on nice and tight.
Hiding in plain sight...