If your gerber does everything you need it to, holds an edge, and can be resharpened to a good edge, then what makes it a non-quality knife?

Back to your question, what makes a "quality" knife quality? I'd say start by saying suitability to to the desired task. If the knife isn't getting you the results you need, it doesn't matter how many features and wunder metal the engineers and sales guys can cram into it. Always start with what you need it to do and the features you actually need it to have.

More towards the answer I think you are looking for, the spyderco endura and delica are "quality" knives that can be had for under $100. They are good quality steel, decent grips over a metal liner and a solid lock. I like the liners because they strengthen the knife handle. A solid lock, positive lock is important because you don't want the blade to close on your fingers. I've seen locks on cheaper folders unlock when griped. The spyderedge serrations can be difficult for a beginner to sharpen. They can be put into operation with one hand and can be clipped to a pocket or inside a waist band. The last two could be relevant for defense, imagine trying to defeat a gun grab. Your strong hand will most likely be on the grip or trying to retain your gun, so you can't use both hands to open your knife. Buck, Gerber, ZT, Camilus, etc. all make similar knives. Benchmade makes a great knife, but they aren't the only game in town anymore.