I like this alot has a lot to look at. I must say that ur man in yellow at the top at first i was wondering what the hell he was doing and it took me to realise the brown thing is a table i'd try making that more obviouse that its a table.

The vent 'rendering' if you used traditional metal shading methods it would be way too detailed for the style of the piece same for the windows. I'm not saying what you have done is perfect coz it does need some work but i think your attept is fine. Too be honest for metal in simple style iso i would have done something very similar.

With the windows at the mo they are white i dont know any windows that are white if you make them translecent so they still have colour to them but u can see the the inside of the building coming through but with light shades that will help with the windows. Then just add a few refraction lines and that will fit with the style.

I hope you find what i said useful even though it is like a essay lol.