I fully agree, all the zombies just feel and behave the same.
to quote my Dear Nelson: My 4.x wishlist (shameless self-promotion, i know):
More Zombie Diversity
I am not talking about the “special infected”.
The regular zombies all look the same in the previous Unturned games and, they all behave the same and are very predictable. This should be changed.The zombies should be more diverse in terms of skin tone/color, faces (currently there’s only one face for the zombies. 2-3 more wouldn’t hurt), hair and facial hair, gender (now that Nelson is introducing a female player model, this will probably be implemented) as well as decomposition (broken/detached/missing limbs, wounds, bite marks and so on) and behavior (some would grab the players and hold on to them and/or try to bring them down, some would slash/bash them as they currently do, some would have slightly, but noticeably different movement speed. Also, some would move directly towards the players, without regards of obstacles and would mostly get stuck on them (fall down stairs, down holes, stumble over rough terrain), while some would behave like current zombies and try to avoid the obstacles).
But perhaps the most important thing is the animation. The zombies should be less coordinated in their movement. They should be shambling towards the players (the way in which they move towards the player should also be diverse), instead of holding their arms up and beaming towards the player as they currently do. They would sometimes stumble to the side and/or fall down.
Receiving damage would influence this. Suffering leg damage would slow down a zombie and/or make him crawl, arm damage would restrict a zombie from grabbing a player, head trauma would make a zombie slower and more prone to stumbling and so on.