1) The first thing is you must have a TDM Allies & Axis spawn and a normal spawn unless you define otherwise. This is the biggest problem I find. The default gametype for CoD4 is TDM unlike VCoD where it is DM! You also need to include a global intermission. You can see all of these items included in the image below.
![Image](http://www.theprogamers.com/tutorials/basic%20mapping/spawns.jpg)
2) This is something that is just highly recommended but the map can still run without this. Under your tools there is a texture called Light Grid Volume. This texture should be applied to all places that the player will be able to walk to. The only cache about using this is that the more brushes it collides with the worse it works. You should use one texture for each room like a block as seen below:
![Image](http://www.theprogamers.com/tutorials/basic%20mapping/lightgrid.jpg)
3) This is another thing that is recommended to do to your map. You should include a reflection probe for every 300² units. This is a personal based estimate that I have found to work good for me. You can see what a reflection probe looks like in the very first image. It is the GREEN cube.
All of these items are found by right clicking in the 2D Grid.
These pictures were taken from a work in progress by me.