I agree with aiko, furthermore i would use a little depth of field, the scaling of your objects also seem a bit off.
There's also a handy tool architects use to save on render time and i believe it is called portal light(hypershade-->create mental ray nodes-->mental ray lights.
It's basically a plane you align with the window which emanates a specified amount of light.(i'm affraid i don't know how to set it up out off the top of my head).
I know that it was a lot quicker then physical sun/sky and such things.
And before you go improve your textures, improve the models first right now you have gamequality models.
-bevel some edges
-the windowframe could use a lot of detailing
-pillows of your sofa need to be rounder-->if you make a poly cube set all off the subdivisions to 3, then press 3 on your keyboard with the cube selected, the cube will be a lot smoother now and behaves more like a nurbs surface, which makes it easy to shape your pillow, when your done reshaping go to modify-->convert smooth preview to polygons.