For me, it's lighting. Lighting and billboards go a very, very long way. It's very important to use a mix of spotlights, pointlights, and billboards for your major light sources. I'm gonna post examples from my own work to clarify.
See how I used a single lantern in that room, so I accentuated all the features in the room from that particular angle. There's a pointlight lighting the area directly around the lantern, as well as a spotlight shining at the stairs so they cast a poignant shadow on the far wall.
Same principle, but this time the light from the lantern casts a clear shadow behind
it, with just a pointlight lighting the area in front of it, since the single pillar is a central part of the room.
The player is trapped in a dungeon, so naturally the retina will be accustomed to the dark. As a result, I placed a very large billboard at the lantern, making it clear that that is the only light source in the room. Also, there is no ambient light in the room, making the contrast between the orange light of the torch and the blackness of the rest of the room that much more obvious.
[Cont'd in next post due to image limit]
This one is harder to see unless you're in-game. First, there are two spotlights present, causing the door to cast a shadow as it opens and closes, and the same is applied the the fireplace. There is a billboard with 'halo' enabled in the fireplace, so as the door opens and closes, it flickers while the shadow is cast. A huge point I would like to make to other mappers is, to create pointlights with low light levels but large radii in addition to the main light being cast. This gives a bounce glow to the rest of the room. It's present in this room, but hard to notice with the white background of this forum.
This is a clear example of the last point. There is a huge billboard in the ceiling, attracting the eye to its origin, but it is still possible to see the rest of the room due to the large radius of the pointlight underneath it. There is also a spotlight with the water gobo at the bottom of the room.
In short, the best way to achieve realism in your rooms is with contrasting and believable lighting. I never let a lamp go with no attached light entity, the default lights Frictional put on their lamp entities just looks bad. They're very bright in the middle, and then they fade to pure back very fast. It looks bad. Real lights go for much farther than you'd think.
Good luck and happy mapping!