It's the textures. They ain't too hot, and they kinda make it look cartoony.
One thing to keep in mind is that it usually helps to have a general theme in your textures- in fact, ideally have only a handful. Things that are built as a unit, such as rooms, tend to get a roughly consistent paint job and decor- you wouldn't have, say, a plate-steel room with bright primary-color paint all over the railings and pipes. Unless you were in a hospital or prison, in which case that'd be cinderblocks, not plate steel, and you wouldn't be going for aesthetics.
Oh, and avoid wrap/parametric/whatever the **** your program calls it mapping at all costs. It's never any good. At all.
