No classes. How good you are depends entirely on your practice and talent. You are put in a box in so many aspects of your life, people telling you what you can do and not do, why would you play a game that doesn’t allow the same freedom?
Diversity comes from maps and objectives.
Deep and complicated game-play, that is easy to learn but hard to master.
Total freedom. You can play the game exactly the way you want and still win. Strong aim, strong movement (in some games), strong timing skills, learning patterns, skills like these wins you games. Some are more reliant on some things than others. The best Quake in my opinion, emphasizes and balances these different skills.
No! Random! Bullshit! The random stuff that happens comes entirely from the physical world. You don’t point the mouse at a given point shoot 5 times and miss any shots, it’s not random, it’s pure.