I think it looks wicked but if you want critique I guess the car's perspective is off a little.
In my noobish opinion, I think maybe it is ok if the humans don't have the same outline if they are considered insignificant?

Yeah, if you want them to be out of the way, then they are fine how they are. If they are meant to be interacted with (such as if this were truly meant to be in a game) then an outline would help to identify them with the other interactive objects. It's just a style choice, really.

I love the piece, it's quite an eyesight.

The one thing I notice needs some work is textures and consistency. There are also a few angular problems with the robot that should have been fixed once you started to shade and define the robot.
Notice how the monsters, car, and HUD are outlined, while the people are not. If an object is to move or is to have some sort of role in the game, it should have similar characteristics as other parts of the game. Also, some outlines appear to be colored while others are pure black in areas that truly make no sense to be so dark.

I noticed on a close review that the light-grey stone wall on the buildings in the back don't quite blend right. The middle sections of wall have no texture, and that really makes them stand out from the rest of the piece, and I don't think that the walls in the very back should really "stand out."

And the most prominent sign of careless pixeling (hard to see from a first glance) is the lack of anti-aliasing and the multitude of banding (such as in the roof corners and on the robot's arms.)

You are a great pixel artist, and I am sure that you can become even better. Keep up the good work!

I think that this gladiator is great, very lifelike and very nice animations.

The skin, in my opinion, is too pink and saturated. Skin is usually more of a dull peach-ish color.

This is what I'm talking about.
My edit is far from good, I just wanted to show my point. The dull color adds for some more realism and allows for the armor to be more of a feature than it was.

Be sure to move the arms and diaphragm when he breathes like that...... It looks empty without that movement.

This looks really nice, and very much like the style presented by the advance wars series.

The first things i notice are the people, who have no definition, I can't tell what is happening with them.
The second thing i see are the propellers..... They don't seem to follow the same angle as everything else.

EDIT: I noticed another thing about the copters, i believe the back end of them would get thinner, right?

I think it may help if you worked on a bigger scale, like advance wars normally did, and then go on to create a downscaled version of the vehicles, it would probably allow you to define the form and shape the way you want to.