There's liberal borrowing from other games. It is no secret that throughout the years, Call of Duty has some pretty significant rivals, and on a head-to-head basis in the early days, the games often came off worse. The PlayStation 2 era titles are a prime example of this, with Medal of Honor: Frontline and Black being far superior to the COD series. It was only with the release of the next-gen consoles, when Call of Duty 3 came out, that it came into its own, having borrowed from the aforementioned titles.
On a single player campaign on Call of Duty, you will be hard pressed to clock up any significant hours of gameplay. You can complete some COD games in a matter of hours. Where they more than make up for this is the multiplayer modes. They are engaging and create competition. I also suspect COD results in many a gamer throwing and destroying their console and controller in anger or frustration. Nazi Zombies, anyone?
Most shooter franchises are hit and miss. No one heralds this game as the greatest shooter ever. Leave those accolades to Goldeneye! The franchise always delivers consistently good games, though, meaning there is a lot of buyer confidence in the franchise. So, rather than wondering if you will like a game or not, COD is a good, safe game to choose.
Lastly, let us know your favorite shooter? (Goldeneye… hint, hint!)