L4D zombies aren't really "zombies". They're more like "infected".
infected. KF specimens actually act more like traditional Romero zombies, even though they're not. The slow, steady, advance and difficulty to kill of specimens makes them more traditionally zombie-like.
Very few "zombie" games have just normal zombies. Classic Resident Evil games have zombies and all kinds of crazy mutants. Left 4 Dead has pseudo-zombies...and crazy mutants. Killing Floor has pseudo-zombies...and crazy mutants.
I guess most people would get bored if the only enemies were just traditional zombies. The thing is, zombies aren't really the "enemy" in Romero's zombies movies. They're more about the bad things that humans do in extreme situations and social commentary in human society in general. Zombies are more of a plot device or a force of nature. Thus they're not just about shooting zombies for the whole movie, whereas most "zombie" games are
mostly about shooting zombies the whole time.
That said, I think a game with just zombies would be fine as long as the gameplay is more about long-term survival, collecting supplies, fortifying, dealing with other survivors, etc, rather than just running around and shooting zombies.