They do what you tell them and if you don't give them orders they attack on their own, but they don't wander off into the distance in pursuit of targets, they stay near you. For those used to the trench-like qualities some mech games have had, MW4 will come as pleasant relief. You can climb up almost anything. There were very few times in the game that I could not take a more direct route to the target if I wished. This gives you so much more tactical flexibility than most games.
Missions come in a wide variety of types and objectives.
There are search and destroy, attack an objective, defend an objective or convoy, capture structures, vehicles or even barges, and many others. Frequently, once you get past the first set of missions, the missions are multi-part. For example, in one mission you have to escort a convoy to an enemy base, take out the base defenses, wait while they scour the base for supplies and then guard them on their way out of the area. Salvage is semi-dynamic. Some salvage is scripted, but anytime you destroy a mech there is a percentage chance that you will get that mech as salvage.