Performance. Oh baby, performance. On many levels.
Oh, and a bit of reliability as well.
The receiver of today will kick the butt of the receiver of yesteryear, and most seperates of yesteryear as well.
Noise isolation has improved dramatically. Nobody complains of radio noise leaking into their pre-amp and amp circuits any more. In fact, nobody really complains about any system sources of noise anymore. Sure a few people have hum and buzz problems, but most of those problems are external, not internal.
Remote controls actually work ... from more than 12inches away from the receiver. Sure, some Remotes could use a little work, but we are complaining about the features, not whether the darn thing can raise or lower the volume.... or keeps the lowering the volume for 2 minutes after you last pressed the remote button.
Power. Lots more power. Circuits have improved, and the reliability of the circuits have improved, and we have more power as the result.
Quality. Take the Big Denon AV 5805. How many isolated power supplies in that baby? What is the rating of those power supplies? Compare that big bad boy to an off the shelf amp or receiver of 20 years ago, even just 10 years ago. Power, Power, and more Power. And better quality Power as well.
Performance is my vote.