Hell, as a purist I also have a problem with running zombies, ammo and guns in every bedroom and how being attacked doesn't result in death (just damage). But then again I'm sure that a purist mod could be easy to do.

Slow the speed of all infected to a walk.
Start of with your primary weapon and just two or three spare clips.
Instant death when attacked
Multiply the number of dead to the Nth degree


Now fight your way through the city.

While at a purist level it would be a bit fun and a challenge, but the current build - it's speed, challenges (special infected), weapon options, etc - make for a great pick-up game that doesn't require tons of thought and rewards you with success if you want it.