It's not really about the engine -- I doubt you would base the entertainment or other values of Command and Conquer Generals: 2 relative to Battlefield 3, because they share the Frostbite 2 Engine?
I think you mean the general gameplay. There's little difference with each one but alas, this is an extremely monotonous topic that has just become malignant and obstinately perpetuated by newcomers like yourself :P
I shall refer to ImTheMostManlyMan's masculine link provided right above me, for further directions in relation to the entire subject of Call of Duty. Please refrain from creating another thread based on said series for reasons including swelling up the amount of unique threads immediately available and creating segregation between conversation regarding remotely different elements that still pertains to the series. It's actually very painful when that happens :P Trying to put forward different points to different people and keeping track or worse, when you need repeat it on a different thread.
No, it won't be any different. The COD series has been going downhill since the release of MW2. Modern Warfare 1 was original and fun the first time. But I can't wait for the new MOH game.
Each new CoD game feels similar to the last. They don't change the game play too much. They add a few things here and there, rebalanced weapons, introduce new perks and other interesting tidbits here and there, but overall the game play mechanics feel the same.
It's perfectly understandable why someone would tire of Call of Duty. However, to say the games are getting worse or going downhill is generally a load of crap. You have to remember that the game is aimed towards people who enjoy the game play elements! It's aimed towards people who want more or less the same game with new twists to keep things feeling fresh.
Let's look at the Legend of Zelda series. Each game is quite similar to one another. The core elements are the same, yet they add and change a few new things here and there. Nobody complains that the LoZ series is going downhill because it's sticking to the same elements that make it great. This is how the CoD games operate.
There's nothing wrong with explaining why you dislike the CoD series. If you think the games have become stale, that's a perfectly acceptable reason. However, it seems like too many people forget that there's a whole audience out there that the game is aimed towards. The video game world does not revolve around a single person!
In short: "This game sucks because I'm tired of it," is a legit reason to dislike a game, but it makes you sound like an anti-fanboy idiot.
Learn to look at the games for what they are, not for what their predecessors were.
If you insist on talking about this nonsense, go to the stickied CoD thread.