Putting in a light_environment entity takes less than a minute and does wonders for your map. Just put the entity anywhere in your map. If you don't know what to fill in for the values, use the recommended values for the skybox texture you chose here
. When you do this, your indoor areas will become dark, so you'll have to use individual light entities to brighten up the space.
I loaded your map to take a look. See this?
The checkerboard texture means your custom texture is not packed into the bsp, and the red error model means the same thing for custom models. If you pack them into the bsp, then people won't have to download all the custom assets used in your map in order to see them. Here's a tutorial
Lastly, making your ground one giant slab is ok for learning, but try not to do this in serious maps. Use displacements and more brushes to make your terrain exciting and not flat.
Edit: one more thing--version your filename using _a1, _a2 etc for maps early in development. If you use the same name for all your versions, it will cause problems for anyone who owns an older version of your map that has the same name. Rename the vmf before compiling or else it might break your map.