Well, I, for one think this is utterly excellent. When I saw it I was seriously impressed. :) I think you did a wonderful job working from your reference. And even though you forgot some things, I mean, your main guy has blue hair - it's a manga car automatically. XD I think most people who even notice the lack of 'things' are those who are looking for a flaw to point out. (And, frankly, I'm not sure you could even see the windshield wipers from that angle.)
As for the flatness.... I think it's all a matter of how you rendered the colors. The characters are cell-shaded, the steering wheel is somewhere between realistically rendered and cell-shaded, the view through the window is painter-ly, and the chairs are sorta realistically rendered. It's a little disorienting when you're going for a seamless "look". I think each of these places is really well done, just that that might be why you're having trouble with the colors. Have you seen what the picture looked like with grey instead of brown interior? (Assuming you still have the full file and can try it, I mean.)
Again - super impressed. :D Also - is that something in the rear-view mirror? For some reason, my brain's processing it as either an explosion cloud or some sort of mecha breathing fire. >> when I intellectually know it's probably intended to be his hair reflected into the mirror with glare over it, it still amuses me very much to think of the other two because it adds a certain... something. >>
Wow! This is very well done. I don't think I've ever seen anyone draw up people inside a car from the INSIDE of the car prospective. Despite the flaws you feel you have in this, I don't think anything stands out as being off. One of the best pieces I've seen in a long while IMO.