I like walkers. As others have said, theyare usually harder to kill, the threat is always there and very hard to escape from. Now if real life zombies where as dumb as Minecraft 1.2 zombies then there would be no threat, in 1.4 you better have your game on.
The problem with slow walkers IMO as we've seen in The Walking Dead by season 3 is individuals even without a gun are able to take them down easily. Andrea for example by season 3 is able to fight off three zombies with a simple pocket knife and take them down.
In order for us to suspend our disbelief and belielve that the whole world has gone to shit with slow walkers becomes more difficult based on individual's skills at killing them.