As a die hard Romero traditionalist, I prefer dumb, slow moving zombies too (in my movies anyway), and find them a lot creepier, however, in the context of this game, I think they're making the right decision to use mostly fast ones (and remember there are
still some slower zombies, too, in L4D, just mostly fast).
Turtle Rock's Michael Booth (the main guy behind this title, aka "God" for me right now) has explained before that they actually did try slow zombies in the game at first, but all the testing was showing it was pretty boring, so they tried the 28 DAYS LATER kind and it totally worked 100% better! That's why they're mostly fast in L4D.
But rest assured, I'm positive people will quickly make "slow zombie mods", that will transform everything over to the, uh, more realistic(?) "Romero" world of zombies. Malls and all.
I think mods will take this game to some very cool levels for zombie movie fans.