Doom 3:Evolution is a game-play altering modification. It messes aroud with ingame stats like running speed, jumping height, stamina and others. That is not all. It also gives you control over implants and tools. These two are used as weapons, however instead of shooting bullets and using ammo- they add to one of your stats like armor or stamina, while draining a special ammo source, or damage your health.
If that's not enough there are changes to old weapons as well. For instance- the Plasmagun and BFG can heat up, and also do bonus damage in the hands of specialists. Speaking of more damage- all classes have a weapon or weapons associated with them. You will also get to carry different ammounts of ammo ( or Junk or Energy ) also depending on your class.
In a few words this is a mod that will change the original game in a way you've never seen before.