What should I do with this background?

[Hudders]

It's missing something, or there's something wrong with it. I'm not sure what. Also: I can't draw photocopiers.

[Monsieur OUXX]

If you want, you can keep it rather cartoony, by adding shadows with no gradients (I mean, add the required shadows, especially the ones cast by the window, but simply as large, darker, monochromatic areas).

Make sure the perspective is absolutely perfect. (for example, the crates on the right seem to be slightly levitating, only because the one on the left seems not to touch the ground, because it's one pixel too high).

Add details to the photocopier (buttons, etc.). Make sure the perspective of the lid is realitstic (at the moment the lid makes no sense)

Round some angles (in particular, round the angles of the photocopier).

[cat]

Nice clean style, but the photocopier is way too clean. Such a thing has buttons and drawers. Google for references.

[Buckethead]

Yeah it's mainly missing a lot of details. Make it come alive a bit by adding lots more stuff.

[Anian]

Already mentioned details and modifications on the photocopier plus details like ceiling lights, blinds on the window, phone, pencil holder, papers on the desk, a few more markings on other boxes, perhaps a trash can with a pile of crumpled paper and when you examine it, it says something along the lines of it being lots of butt prints from the last office party or something. And maybe a poster or two on the wall, from a kitty to motivational posters or a simple calendar with a pinup girl etc.

Maybe experiment with putting something in that black door frame, even a small hall and cubicle wall would do, just leaving it blank seems a bit lazy.

[TheRoger]

Hmm...I don't know what kind of style you're trying to get, but I'd make corners darker and I think I'd add some soft textures or walls, boxes etc. because they look too clean.

But that's just my opinion. 

[gameboy]

What bugs me a bit is that some details (chair, boxes) have darker outlines which kind of clashes with the rest of the picture (no outlines). Also, add some details 'n stuff as said before, plus add depth to the window and door. You might also try to increase contrast (eg between the darker and brighter shades of the boxes)

Overall you have a nice clean style and good use of colours.