I thought Marcus was a fantastic character, and I found myself rooting for him more that John. Would have given some interesting twists to work in for future films.
I agree, but Marcus died, and the series will degenerate into mindless action.
I like the part where John was going to face a terminator, and he desperately tried to jump start Marcus' heart, so Marcus could take on the robot. Kinda funny, in a morbid sense - "hey, revive yourself, before it kills me."