It's something that has become clear to me after hours of gameplay - the 50% rule should absolutely be added. They can make it their own by tweaking when/where it comes into effect, but it's clear that the gameplay suffers deeply without it. The crazy thing is, I didn't even notice it until it was missing. Now the levels feel unresponsive and unreactive... they feel like a slow-moving conveyor belt of enemies.
Zombies shouldn't be coming in one at a time. There shouldn't be single coneheads spawning and then five seconds of nothing. The level needs to react to your gameplay. The better you do, the faster the zombies should come. And after a punishing wave, you may need more time to recover.