Okay, first of all, it is possible to have many very different subjects in the same style. Case in point:

There's a rock, a person, a robot, an alien, and a store all in the same style. I'm not saying that yours aren't in similar styles, but there are differences. For example, the sword-wielding protagonist-looking character sticks out like a blue thumb.
And if you want examples of metal doors, google is your friend. In fact google is always your friend. I found these pictures in the first two rows of a google image search for "metal door" XD
http://clarsen.net/multimedia/1012_DSCF0156.JPG~640x640x85.jpghttp://www.schneiderlocksmith.com/images/product/metaldoor3b.gifhttp://www.pixelpoppin.com/vapor/newSurface/surfactpictures/albanydoor.jpgIn case this was not the kind of help you were looking for, I will attempt to give some crits on your work.
-You probably can see this, but the hero guy needs more detail. He is incredibly flat, while the robot and alien have very nice shading rounding out their forms.
-Why do you have so many colors on the walls? The lights' cones could probably be drawn with less than half that many colors. I didn't even see half of them until I zoomed way in.
-I like that you put some variety into the walls by knocking one of the lights out and breaking the wire. However, you could get even more variety quite easily: try making one or two of the grates messed up somewhat, or even put stains on the wall or something. You could also get some variety by putting cracks or something into the ceiling and floor.
-Speaking of the ceiling and floor, what's up with them? The rest of the world appears to be made of metal, very high tech and stuff, yet the floor and ceiling seem to be made of adobe brick or something. I don't have any suggestions off the top of my head for what to do there, but the adobe looks very out of place.
Now for the door.
-First of all, the light is not aimed at the door, it is coming from overhead. Thus, the highlight should be more horizontal. The middle reference pic I linked to shows a good example of horizontal highlighting. Try doing something like that if you can.
-I do like the line dividing the halves of the door; you did a good job defining that. However, since the light is in the middle of the door, you probably want to reverse the lighting on the bottom vertical part.
To sum it all up: overall, it looks nice, especially the sprites. The background could stand to lose a couple of colors and perhaps have a bit more variety, but it is quite well done. The door has nice lineart, but the highlight needs changing. I look forward to updates!