Yeah, you really shouldn't give opinions on games without talking about the campaign mode. however it's true that as far as multiplayer goes, it's essentially COD4 with the weapons scaled back to suit the time period, and the game looks pretty damn similar. but that tends to happen when you release two of the same style games without any tech advances between release. They released them with so little gap in between (in fact, almost exactly a year), they didn't have time to change much other than the campaign and how old the weapons are.
To everyone who says they can't be compared because they're different games and blah blah blah. Yes, They can be compared. Not because they were made by the same company, but because they're almost the same game. WaW doesn't even seem to be it's own game, more like an add on or mod of CoD4. The actual gameplay is almost identical, even if the maps have been switched.
Also just to shut everyone up about who developed L4D, directly from the Steam L4D info page:
Title: Left 4 Dead
Genre: Action
Developer: Valve
Publisher: Valve
Release Date: Nov 17, 2008