It is about killing zombies, they won't have a organized/cover AI system like if you were fighting specialized trained soldiers (though some of the special zombies do run away for cover, jump around). Also they have almost no long range stuff, they are zombies so yea, you don't need much skill to kill the zombies. The skill comes from working together, a few zombie hits does knock you down so you need your team to help you.
I find the versus mode to be very weak, the switching between survivors and infected looks like a bad choice to me. Should be survivors vs infected throughout the whole stage, not revisit stages by going back and forth to other sides. I love the campaign however, with few more maps the game can really be played for months and months but as it is now you may get tired of it after one month.
Also the game is strictly an online game, single player shouldn't even be counted. Each map is like its' own world, you get saved in each map so the game really ends there, they were going to connect all the maps and explain why you are on the next map and not saved but they scrapped that idea- just an online game.
