We really like the style and composition of this - mixing the 2d and 3d works really well and overall there's a lot of great things happening here. You demonstrate a good understanding of the principles of animation, there's some nice subtlety, overlap and weight. Things we think you could improve are:
(1) whats the man's motivation? this would determine how he lifts his head, but as a general rule I would offset the head rotation relative to the chest/body - as he looks up the head should lead the action.
(2) adding a small amount of overlap on the head as the body weight goes up and down would convey more weight.
(3) mans left arm is a little static and could swing more as he steps towards the door, having the starting position closer to the door would give the arm somewhere to swing to.
(4) the angle of each foot in the passing position is almost vertical, I would consider reducing that to a more natural position.
(5) the left foot sticks slightly in the passing position on the first step, I would smooth the rotation out to reduce the amount of time on that pose.
(6) the dog last few steps with the front legs seem slightly overstretched.
(7) the last step has a slight hesitation and the passing position is a little high. I would reduce this.
(8) the dogs head becomes quite static at the end - what's his motivation, he could be sniffing around.
(9) there is a rocking horse quality to the body which needs to be fine tuned - look at some reference of a running dog and analyze.