Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
The "Squad Leader" Gamescom trailer for Mass Effect 3 is only a minute and forty seconds long, but it contains a wealth of information about what it calls "Combat Reveal Part 1". Commander Shepard leads a squad of three, with Garrus and who I am fairly confident is Liara (unless it's Samara in a lab coat), against a group of Cerberus soldiers (again probably indoctrinated) and one badass Atlas heavy mech that interrupts their fun by falling from the sky like a comet.
From the very moment Shepard enters the narrow, dirty alleyway, you will notice that his sprint speed has increased. You can still assign cover to your squadmates and go into cover yourself behind conveniently placed and conveniently indestructible crates. Shepard can vault over crates as he has before, but it's not certain whether Shepard strangely needs to crouch behind cover before he can vault over it.
Shepard has access to a chilling Sub-Zero projectile blast that Garrus can exploit with a well-honed sniper shot to a frozen opponent, as well as a secondary heavy weapon that seems to have homing, perhaps heat-seeking ammunition. With a quick roll to the side, he discovers a Cerberus officer attempting to find cover and whips out his new omni-blade for a melee death strike of fire that is far more effective than punching an awkward number of times.
The Atlas heavy mech, as usual, has ridiculous health and shields, and a wide arsenal of missiles and short-range one-kill death shots. But with enough overloading and grenade abilities, it can be worn down until it gets blasted into smithereens.
I still wish that Shepard could lead more than two squadmates or that there are more team-based missions like the finale in ME2. (I mean, what are they doing on the Normandy that's so important?) There's also a minor but significant change in how Liara uses her singularity power, which from the looks of it, forms at a point in space rather than through a projectile blast. Hopefully, this means that biotic abilities don't get blocked by some dumbass box that gets in the way of a miniature black hole.
But from what can be gathered from the trailer, Mass Effect 3 looks to have an improved combat system that doesn't change too much of the core system already in place from Mass Effect 2. Mass Effect 3 looks well on its way for its worldwide release on March 6th, 2012.