Pros:

Doom textrues
Secrets
Nice map/design
Works good and without bugs
Endless waves at the end with infinite ammo :D

Cons:

Only one Map
No Doom music
Too easy on Expert, its Doom, make it fu**ing  impossible :)

Conclusion:

I rated ten, because I love Doom and I love L4D and this is Epic !
Please make the complete first campaign

And sorry for my english, I know its not perfect ;D