The only thing you are really missing is some decals. Your walls look too plain and need some decal cracks or some kind of other wear an tear. For instance in your last picture in the first post, I don't notice any tiling, but its just repetitive because your left/top/right walls all look the same. For instance look at…
Thanks very much for the nice feedback and advices! So here is the final scene (added some decals and changed the lighting a bit). You can also see the used geometry and textures ;) The whole scene's polycount is about 170k tris (a bit heavy, i know ^^) Let me know what you think!