I would like to see Zombies being clumsier than the player and taking a bit longer to climb over fences/through windows etc. If you think about Romero Zombies, they wouldn't probably even be able to get through a window without falling on their face. I think it would be a good compromise, immersion wise, to have them slow down a bit. This way there'd also be a use for fences, barbed wires and sandbag walls. They would actually help in the defense of your safe house. Maybe even so that fences were the most easiest to climb (both for the player and NPCs/zombies), then sandbags and then the barbed wire (simulating your clothing getting stuck a bit). I agree wholeheartedly its always bothered me how they just jump over walls without hesitation.