Here's some specifics.... Tier 1 rifles, like Colt, BCM, LMT, Noveske, Larue, JP, etc, have things like:
-Properly staked bolt carrier gas keys that don't vibrate loose
-MP tested bolts and carriers (higher likelihood of not crumbling into dust sooner rather than later due to metallurgical flaws)
-Better quality control (When's the last time you heard of someone that bought a Colt that was a piece of s**t out of the box? Not that it doesn't happen, but it's pretty rare)
-Barrels that aren't s**t. They almost all use 1/7 or 1/8 instead of the 1/9 fudd twist, and on top of all that, better barrels are more accurate with more bullet weights.
-Approval by pro trainers and guys who kill bad guys for a living (Yes, this isn't technical, but IMO this is worth a lot to me. Given a choice between some random thing and a rifle that a pro is willing to trust his life to, I'd rather use the pro's choice. )
This isn't to imply there aren't a lot of Tier-2 rifles which aren't damned good values, eg, S&W, RRA, etc. It's just that when you go out of the top end, your trust of the rifle should be speculative at best.
There is stuff at the bottom like Olympic/Hesse/etc. that is basically best avoided. A commercially made rifle for $600 = run away, fast. Quality control is nonexistent and corners are cut at every turn on those guns. They are junk masquerading as an AR-15 clone. Olys are like the fung wah bus... there's a reason the tickets are so cheap. Don't be surprised if the bus catches on fire on the way to NYC.
There is a bunch of other no name junk in this price class too, same deal. You will know it when you see it.
If you need a rifle for that cheap you are far better off building it yourself. You will save money and end up with a better gun.
-Mike