there's several things to make it look more "realistic".

first of all, when you can waste some more polies, try rounding the edes of the pillars, maybe cut away some bits too in that...

now the textures/materials:
The doorpost looks incorrect, change that to something that fits the environment (search google for reference pictures).
Another very important thing missing there is blending of the textures, if you look at the floor for example, good image, wall, good image, together.. too blend.
Try making the wall texture look more worn, you can see a hard edge on the bottom, towards the floor, try blending the texture with a dirt mask towards the ground...
If you wanna add some cool new effect, make dirt sprites and randomly place them over the ground, makes the ground look different as well, and in general look a bit more interesting.
and as said, the ceiling looks waaaaaaaaay too clean!

without bragging about my own skills, because they are far from great, you can still search for the sewer textures i once made, see what i mean with the added dirt everywhere.

regards,