I think L4D is over-hyped, not because of the developers but people in general are going nuts.
It's still going to be pretty good i'd imagine but people arent going to be so amazed after doing the same levels over and over again.
The AI Director makes the zombies spawn in different places every single time, so you'll never be able to know exactly where they come from.
Its not what people mostly think it is, its CSS weapons with HL2 Zombies (Plus a bit more), co-op game. Its nothing amazingly special other then 10 zombies with out any lag.
Sure, they reused some CSS weapon sounds but the only weapon that's really like a CSS gun is the M16. Also, HL2 zombies != L4D Infected.