What parts do you see problems with?
Some of it's the parts, and much of it is the assembly. They seem not to know what staking is. DPMS & Bushmaster are the two brands that I consistently saw undersized chambers on when I sold guns. DPMS' more basic models have cheap stocks - pardus buttstock & glacier guard hand guards. RE castle nuts not staked, carrier keys not staked. No HPT/MPI on bolts, nor barrels - not a big deal a few years ago, but now other companies do this at competing prices. You've posted several of the other issues I've seen such as:
Other than a few gritty triggers and weak extractors the ones I have tweaked seem OK.
Are you kidding? If it wont extract, and I have to tap brass out with a cleaning rod, I might as well be shooting a muzzle loader. What's the point of a semi auto that won't extract? I don't want an OK rifle, and especially not at $600+. I want good at least. Is a chamber that's cut correctly, and an extractor that works too much to ask for in any rifle?
I have seen some pretty loose butt stocks on what seemed to me to be undersize RE's.
Seen that too. You go to replace their cheap stock with a standard M4 style stock, or a nice aftermarket stock, and both mil & commercial ID stocks are loose. Is using a correct RE - either of the two established standards - that much to ask?
I haven't seen any one thing on them that's really that bad.
So, other than the fact that they often don't work properly, DPMS AR-15s aren't so bad.
Like I said, I like my LR-308. I never saw the problems with the DPMS LR platform guns that I saw with their AR-15 platform guns. If they'd apply the same build quality to their AR-15s that they apply to their LR-308s, DPMS AR-15s would be solid rifles within their price points.