Running around in third person is one thing, you know that your player is in the middle of the screen. Detecting enemies from a distance without the forsight to know where they are is another completely.
It looks like you've used a base light ambience of 200, which is the default setting. The average light brightness in TF2 ranges from 450 to 1600. So you should at least double your maps brightness, if not quadrouple it or hectople it. You should be aware that map brightness is calculated using light bounces (20 to be precise) and certain textures reflect more light than others. Brown wood reflects significantly less light than white bricks. Gray cliff rocks will also reflect small amounts of light making your map darker than the base light values would imply.
Take Valve maps as a status quo (because they are), doublecross is an example of a dark map. The lights still light up the play space so that players can see each other and determine threat levels from the enemy class and weapon loadout. You should follow suit, even if you think you know better (which your experience would imply you don't). The guys here are seasoned TF2 players and seasoned Hammer users, you should listen to their advice.