People keep drooling over the big mechs, but what I recall from the MW3 multiplayer was hordes of Pumas running around and blowing off the knees of bigger mechs. The most common loadout for big mechs I saw was the missile salvo loadout. It basically knocks a mech down from the sheer intensity of the blast of a thousand missiles(okay, actually 120), immobilising a mech for a direct killing-blow.
All the trailers just seem... slow-paced.
Cheers.