The Call of Duty Problem

The continuing Call of Duty series suffers from three main problems:
1) Repetitiveness
This is not just confined to one specific area of the game. How often can you take the same setting, the same battles, the same situations, the same weapons, the same screenplays, the same levels, and repeat them in each game, and then make five games exactly the same? With Call of Duty 4 being the exception, all the Call of Duty games have been set in World War II. Many of them contain the same battles (Stalingrad, Berlin, etc), and have you doing the same things as in the other games (raiding houses, shooting people, not being allowed to open doors, waiting for your slow comrades to continue). Each game virtually has the same weapon list since the beginning (Thompson, M4 Garand, Mp40, Sniper, etc) with few exceptions. Wrap it up in the same generic levels of "shoot respawning enemies from a variety of locations in a variety of environments all while your teammates are useless," look down on the battlefield from below in a plane mission, get on a tank in a vehicle mission, drive a car in a chase mission, and then get giddy in the good ol' grab a sniper rifle and shoot limitless enemies from a battlefield safe location. Even the individual games repeat their own missions and gameplay concepts, so how does it make sense to repeat those games over, and over, and over? It already gets tiring after the second game.
The FEW--and I must stress FEW--exceptions to the Call of Duty formula are where the games truly shine. In the first game, there was no established formula to expect, so all the situations and ideas were unique and fun. In the second game, the formula was repeated, but they still had enough variety in the missions to maintain a level of interest. The few things that Call of Duty 5 does differently (flamethrower, solo sniper missions) are the only interesting scenarios in the entire game. Looking back, it's been that way for each Call of Duty game, since the standard combat formula that constitutes the majority of the gameplay is just plain repetitive and boring. The unique missions where you do things other than just kill respawning, endless clones of Germans are where the most fun lies.
When a game sticks to a strict formula, it might work the first time (because you don't notice it), but then it ridiculously hampers enjoyment from then on if it is the meat of the entire game's core--which it is, in Call of Duty's case.
2) Lack of Strategy
The Call of Duty gameplay formula for individual battles goes something like this: Confront the enemy, shoot him with your rifle from a distance, advance on him by rushing to cover spots, throw a grenade to kill him if you can't shoot him, if in close quarters use an automatic rifle, if in really close quarters use melee, and finally... when your enemy is dead move on to the next enemy.
You are always given the same weapons and the same environments and the same enemies. You approach every battle with the same mindset, the same strategy, the same plan to what you are about to do. This reduces the game to a chore--a "to-do-list" of things you know you have to do. You already know what you are going to do, how you are going to do it, and there are few spontaneous events that truly force you to adapt to a situation and use the game mechanics at your disposal in order to solve that problem.
This is what separates games like Bioshock and Team Fortress 2 from Call of Duty. The first two have you constantly adapting, thinking, and analyzing every new combat situation in order to get out of it. You are always engaged in the battle. On the other hand, in Call of Duty, every battle unfolds the same way, with the same set of tools to overcome it. There's no strategy here. Just mindless reflexes, timing, and pacing--which is fine if done right, but Call of Duty has just repeated it so many times that this mindless combat can't survive on its own.
3) Lack of Challenge
Games should be challenging. On one end of the spectrum you have the "Win Button," where the solution is in front of the player, and all he or she has to do is click it. On the other end you have whatever you can imagine to be the most frustrating, inane, and difficult game you can ever conceive of. Games (and the word "challenging" for that matter) should fall somewhere in between the two.
Call of Duty has progressively moved into a zone where it's not challenging because of the design of its combat. You are always given the objective to kill the random Germans, and you are told how to do it, where to do it, so all you have to do is go out and do it. Everything just becomes a matter of when--not how, where, or why. When is the endless respawn wave going to end so I can move on to the next arena? When am I going to have to use this Panzerfaust on the inevitable tank that will appear? When am I going to finally kill enough Germans for this area to be designated "clear" and this mission victorious? It's not how am I going to do it, or why am I going to do it, or where. All of these things are given to you, so the game just waits for you to do them.
This is not a matter of challenge. It's more like a matter of how fast can you beat the current level, rather than how do I figure out how to destroy these enemies in the most efficient way?
Without this challenge, the gameplay becomes stagnant and unmotivating.

Call of Duty 1 was a great game. Call of Duty 2 was just as good, except it was more of the same. Call of Duty 3, I will not talk about. By the time we hit Call of Duty 4, this formula had been repeated too many times, with too little variation. Call of Duty 5 was just worse.

Many of today's successful franchises rely on repetitious formulas. The Zelda or Half-Life series, for instance, are prime examples. What those series do differently however--for the most part--is disguise the repetition through different environments, storylines, and characters. Call of Duty currently lacks that, and because of this, its future as a series looks like to be more of the same. Unless it can solve these problems, each game will progressively become more and more generic.