Any tips and tricks/ advice on improving the grpahics of my game?
Use more then one texture. Use normal, specular maps. Make custom shaders. Add detail objects like pipes/wires/access doors/garbage. Simple stuff really.
Thanks for help jeffro11, just one more thing. specular maps? could you explain
I agree with jeffro. I would add more objects to the scene. Wires, trash, pipes, or even just decals of trash and dirt.
Normal and Specular maps man…trust me, those are the #1 thing you should do (and then yes, of course extra detail objects). Fortunately, if you don’t know how to make them externally you can make quick and dirty ones directly in Unity 
Change the shader of the material of the wall to bump specular. Now select your diffuse texture. Find and check the box that says “Generate alpha from grayscale”. Hit apply and ow your diffuse texture has a specular map.
For the normal map, make a copy of your diffuse texture and switch the texture type to Normal Map. The box that says “Generate from grayscale” will be turned on by default. Then adjust the bumpiness to whatever you want and hit apply (getting the right amount of bumpiness will require a little bit of trial and error since you’ll generally want it to be subtle). Then just put the normal map into the normal map slot in the material using bump specular shader.
To get best results you’ll want to make the normal and specular maps in an image editing program like GIMP/Photoshop or programs like ShaderMap Pro or Crazybump. But for a quick 1-minute job this’ll work well enough.
Also, you’ll want to take a look at some of the other features Unity has. For example, lens flares might look good on those lights in the corner. You could make some particle systems close to the ground to look like mist or make water dripping from the ceiling or steam coming out from a broken pipe…
Line renderers are also useful, they have lots of wonderful applications…
The only problem I see with your screenshot is that it lacks global illumination or ambient occlusion.
Other than that, i would add lighting. Thats important too. And you can use lightmapping too, Does makes the walls nicer.
Looks like other people have covered a lot, but it also looks like your map has sharp looking texture seems or hard normals. For angles that are straight, make sure that your normals are smooth on them and in your UV layout make the textures loop better. Of course, I agree with everyone else’s advice that you need some different textures then just that one concrete texture, and you need to give your textures a bump map and (maybe) specular maps (the reason I say maybe is because concrete isn’t really shiny by nature). And, add more stuff to your scene unless you are trying to make it look barren, in which case you need to work on your lighting, make the textures look more dirty and out-of-use through decals or something, and add some fog (maybe you should do that regardless, fog looks nice and is a good LOD technique).
If you want an excellent tutorial on using the right shaders in the right environments, check out i3Dtutorials.com’s advanced Unity course.
(Although for most of the part you need a Pro license)
I’d change the floor texture, it’s the same as the ones for the walls. Also normal maps and Beast could help.