hello you
if i understood you right (i am german and not very good in english), you want to know what are important things which you need for a good level.
ok, i think very important is the lightning. it doesn't look fine, when you only have 2 or 3 lights in one room, because then the shadows are too sharp(?).
you should place few weak lights in the shadows so that it looks like the objects in the room shine a bit,too (difuse light?).
also important is texturing. for example the floor texture (for example tiles):
if it looks like the half tile is in the wall, it doesn't look very real.

hope that helps,
bluecat