Respond quickly and accurately to ATC instructions. I trust a pilot way more when I see the plane turn while the readback is occuring...
+100
Pilot knowledge and skill on the network sucks compared to what it was years ago. When I joined, you could count on at least 85-90% of the pilots to fly well, and most of the rest were really interested in learning to do better, with the occasional bonehead thrown in. Now the numbers are significantly different. I was flabbergasted watching last night's FNO, where indeed two arrival runways were being used, yet roughly 30-35% of the pilots couldn't execute to save their lives, with WAY too many that simply didn't even know their left from their right. (Side note for pilots: NEVER EVER when a controller is asking you why you went left instead of right say, "well, I dialed in the heading and the plane just took me left...". YOU are supposed to be a pilot, one who COMMANDS an aircraft, not a MONKEY who turns a dial, grabs the popcorn, and just WATCHES what's happening to him, screaming "WOOHOO!")
In most cases, a controller can manage and recover from a bonehead. The problem really occurs when there are two or more boneheads (or even one bonehead and one or more "less skilled" pilots) in a row or at the same time that the controller is trying to deal with simultaneously. That ramps up the stress and ramps down the efficiency tremendously, and can quickly and easily cause the controller to get behind, negatively effecting the experience of the controller and several pilots. And unfortunately during large events, it seems to happen more and more....