First off, it seems as if your textures are competing against one another more than co-existing in harmony and that kind of kills the " game atmosphere effect". You need to soften at least one of them a bit (maybe recolor one of them with colors that are slightly more similar to the others so the bricks by example don't look like they're protruding from the wall so much but like they form part of it?) and maybe darken the wall tile closest to the floor line-- I see some pillow-shading going on there.
Edit: You don't need to draw every rock or every brick to get a good-looking texture;
this thread might help.
I like the variations in the brick wall so far, but maybe you should make their shapes more different as right now they are of about the same size and look very similar. The circle-pattern detail at the top looks off to me; it is very unclear how it is connected to the wall and what it is (something like the roof sliced at the middle? Some detail placed at the top of the walls?) they are especially confusing because they give the idea of it being a square room, while I believe sewers are mostly rounded tunnels (but I'm no sewer expert so whatever).
Even though the rock floor tiles "OK", I think they look a lot more like metal scraps than like rocks. You should probably look at more references of stone floors. The door also looks suspiciously not rotten for a moist place such as sewers! And maybe you should hint in the wall that there is a door here, you know, so like the bricks could form an arch there or whatever.
Finally, the picture really needs some hue-shifting, it's basically all plain shades of gray. Maybe you should try several blues and greens? Look at real pictures, use different colors for different things to create atmosphere!
