Just got to chime in here about COD4, i am not sure why everyone says the game is too short, if you look at every COD game they are about the same length, the only difference is that COD4 is structured differently.
In the first 3 on pc each level is broken up into many different parts, get to bridge, clear bridge, cross bridge, attack bunker or similar. thats 4 separate levels in the game, where as in MW its listed as one level.
So when you look at the level select screen in 1 to 3 theres 30 or 40 levels, but in COD4 there is 18 or so. the levels are basically the same size overall, and in COD4 there is more freedom to move around the level as you want to instead of a “path” as in the other games. I do not agree with the statement that it is close to “on the rails” unless you choose to do it that way, COD5 is far more on the rails than 4. the only thing lacking in 4 in my mind is a bonus level in the mind set of nazi zombies, because the replay value on that is immense, and the aircraft level gets old fast.
Now that is out of the way, i also do not think 10 hours of fast paced FPS action is a problem, that seems like enough to keep us busy waiting for the next one, and as stated above older games were far shorter in length, but due to annoying deaths, could take longer to beat.
And last but not least on my quasi rant i havent played fallout 3 or bioshock yet, which depresses me insanely. But the RPG element is sure to increase the time spent playing and give you more bang for the buck, but when you just gotta unload a airstrike, you can always find half a hour out of ten hours of gameplay to get the juices going!!
It all depends on the game and the player, if the game is fun and well designed, the time spent can be endless, i still hammer away at GTA3 a few hours a month.