I finished the first two Gears (PC for #1 and I definitely appreciated the expanded elements in the PC version, such as the Brumak encounter; 360 for #2). They were enjoyable, but there were nothing in there that I can recall as phenomenal level/encounter design and any mechanics beyond cover. I played both in co-op campaign, with a friend, and have never replayed the campaign, nor loaded the multiplayer ever, so take the below with that in mind.
I can remember various encounters in the game, such as the General fight towards the end of #1, the Brumak mentioned, the end encounter of #2 (which was very lame and easy beyond belief), the giant strider beasts you get on in the Locust prison areas, the Queen (and man, that was lame), The flood-infected locusts...maybe T virus...no, wait, "Imulsion" ;-) However, none of them seemed different than tons of other games of this style. There was no moment I felt they did something special or above and beyond what is formulaic for the genre.
Instead, I found each encounter to be highly telegraphed and from a very narrow angle of attack. I would just find cover, because you knew when and from where an attack was going to happen and then wait for them to arrive. The set-piece environments are generic "abandoned industrial/gothic/ornate" or "cavern".
The mechanics, which you - correctly - stated as simple are hardly deep. You cover, heal rapidly, pop-up, shoot. With 2 guys you can easily cover the entire angle of attack in an overwhelmingly large part of the first two games. I can't even imagine how 4 players would find a challenge. In co-op, my friend and I played on auto-pilot for the first two games, because it took so little thought and offered so little surprise. So where is the depth? I never ran out of ammo and only ever switched a weapon if a heavy locust dropped something I wanted, like the mortar gun or what not.
The AI does nothing that I recall as good; they let you shoot them, they don't work as teams, flush and flank you or anything else that might be the "I" in AI.
Where is there friction? You mean in the story? There was rarely any tension in actual combat; just shooting, healing, shooting.
Again, I enjoyed the time I spent in the games; they were bug-free, polished, ran cleanly, but they are "popcorn cinema"; you tune-in and tune-out. There's nothing deep or original, just good fun shooting things (or using a chainsaw-gun), explosions and being the Good Guy. I'm perfectly fine with that in a game, but I don't pretend to say it's the King of any Castle just because the development or budget is one of the largest in the land.