Ok I have a few questions for all of you winging about games using the same engine...
Have you played any of the Splinter Cells? How about any of the Rainbow 6's? Ghost Recon? Gears of War? America's Army? Bio Shock? Mass Effect? Unreal/Unreal Tournament? All on versions of the Unreal engine, which has been incrimentally updated for the last 15 years. How about FarCry2, the Dunia engine? Modified Unreal engine.
How about the origional Half Life, and all of it's mods? All on the Gold-Source engine, which is a modified Quake engine.
Then we have the Source engine. The origional was released, and then there was the Ep. 1 revision, the Ep. 2 revision, on which Ep2, DOD:S, and TF2 all run (Do these games in any way feel samey?) Left 4 Dead uses a further modification of the Source engine.
The game mechanics are what make CoD:4 and CoD:WaW feel so similar.
In summary, you silly boobs, the engine does not determine the game.
That said, I hope CoD:6 takes a lot from 4, what I'd like to see improved are balistics behavior. (All weapons being perfectly accurate, bullets penetrating nearly all materials, stuff like that. I thought the perks were a nice idea, but I think they should be more subtle. It's a nice way to lean towards a certain style of play, such as stealth, or carrying more weapons, more armour, or the like, but bullets penetrating 5 inches of steel like it were wet toilet paper is too much. (IMHO)