this is my theory on why mw3 has became boring.
the map sizes of mw3 and blop were reduced down. A small map for cod4 lets say destrict has became a large to meduim in mw3. So with maps being smaller this pushes the spawns closer together, so now instead of having 5 to 10 seconds of movement before you have some sort of engagement you now have 3 to 5 seconds. The game play is faster and theres more action for everyone involved. The problem here is that with the smaller maps and closer spawns you are having exact same engagements every game.
You spawn in, you go one way knowing that someone will becoming the other way, one of you's dies, now repeat the process. This is usually the format for how people play in any mutliplayer shooter, but mw3 has no cool down period between death, spawn and action. You die, spawn in and start shooting at someone in 5 seconds or less. But unlike in other shooters the fighting takes place in the exact same spots every game, every map, every game mode and there's nothing anyone can do because you spawn right beside the action and can't move anywhere else because the maps are so small.
