Hmm another good question. The missles on my MW3 don't lock by radar alone, I still have to target my target and face him. But the locking area was greater in MW3, you didnt have to be aim dead at the guy like in MW4
Well you had to be in the targeting box, but in MW3 you could lock-on over a hill without a line of sight on the mech, where in MW4 you had to have a direct visual of the mech to lock on.
I love the combination idea someone had on these forums. It makes sense that the arms of mechs should be able to move and shoot something off the center torso, but the hard mounted weapons on the torso (think Fafnir gauss rifles) must fire straight ahead no matter what. Someone suggested both types of reticles, a MW3 free floater that would control the arms and other moving weapons on mechs, and another faded reticle that remains in the center of the screen at all times for the hard mounted weapons.
The free floating reticle in MW3 may have made it 'easier' in campaign mode, but multiplayer was a level playing field and extremely challenging. Either way in MW4 I mapped torso twist to the mouse x axis and never looked back since, although I still love the floating reticle.