1. It was a rail shooter for PC only
2. Aliens invaded earth, so you had to travel to their worlds and destroy their stuff
3. you had a portal like device that sent you from world to world
4. there were parts where you could turn the camera 360 degrees
5. you had an ammo limit and a grenade limit.
6. some enemies outlined in red would instant kill you if you didn't shoot them fast enough
7. your base was in Washington DC and that was also the final level.