flat top upper
quad rail handguard
4150 steel for the barrel
proof round fired
magnetic particle testing on the bolt
forged front sight base
Those are the big things to look for. I used to be able to tell you which companies to buy from, but that was a few years ago and now everyone and their brother makes AR's. So stick with Mil-Spec and you'll be OK.
OTOH, I CAN tell you a couple of companies to absolutely avoid. Either because their quality is sub-par, or you won't get the barrel twist and steel you want, or they don't MPI their bolts, a combination of all, or something else.
The biggest ones to stay away from are M&A Parts, Model 1 Sales, and Rock River Arms. I'd throw Bushmaster in there too, at least if you're not planning on swapping the barrel out right away (they don't do 1/7 twist).
With AR's, generally you get what you pay for. LaRue is expensive as fuck, and I've heard the guy is a real douchebag to deal with, but everything I've heard about the quality is positive. You pretty much can't go wrong with anything made by Lewis Machine & Tool, Bravo Company (BCM I think? Google "Bravo Company AR-15"), or CMMG. Colt and FN actually make the rifles for the military, so they're obvious choices too.
With your budget, I'd suggest buying a stripped lower, then go spend an hour on AR15.com learning how to assemble it. Buy your lower parts kit (you can get them for like $40 or something at Cabella's, probably cheaper online somewhere) and the stock and pistol grip you want. Then go to either LMT or CMMG's website and have them build the upper. Select the options you want, they put it together and ship it. If you don't know what the various options are, Google is your friend, as is AR15.com.
I personally like free floated quad rails. My next upper won't have the standard front sight base/gas block. And I may consider one of those monolithic rail platforms, where the upper and handguard are all one piece. That's probably a bit out of your price range right now though.
One name to remember: MagPul. MagPul, MagPul, MagPul. They make magazines, pistol grips, handguards, stocks, sights, all kinds of good stuff. Both of my AR's have MagPul stocks; the M4 has the CTR and the A4 has the UBR. The A4 also has a pistol grip from them. And my SCAR is currently sporting one of their angled forward grips. Speaking of forward grips, don't waste your money on cheap shit if you're going to get one. Other than MagPul grip, I have several by Tango Down, and they are solid. Good stuff. They're also somewhere around $70 a pop.
Yankee Hill makes good stuff too.
With $1500, you can build a pretty decent AR if you build the lower yourself and have someone like CMMG build the upper. You could do even better probably if you built the upper, too, but I've never done that so I can't tell you anything about installing the barrel, headspacing, etc.
Life is a game that you lose by playing by the rules. Luckily, the only time it's too late to start winning is right at the end.
"Save yourself, never go back into a burning building or a burning society."
"Battles leave scars... some you can't see."
-- Kanan Jarrus