First off, in isometric, it generally looks better if you either highlight the edges that stick toward the viewer (see the top of the walls) or don't draw anything on those edges at all. So there are 3 lines on the desk that should be highlighted, and each door has a line of highlight that I don't think should be there.

 

You have a pixel miscolored where one of the floor gridlines meets the edge of the cutaway wall on the left. The computer screen is 2D; give it a little bit of thickness. The man's wearing VERY bright clothes, and generally your colors have very high saturation. Desaturate a bit and it should look nicer. If the doors are wooden, they could use some wood texture, and the cross on the floor is missing highlights on the back part. Maybe you should also shadow some of the bevelled bits there.