Wrong sir, as an FPS, most classes take skill. For demoman you have to judge distances because of gravity, and you need to be able to estimate distance for lobbing stickies over walls.
Sniper is like any other fps, so it depends on your general skill in aiming
Soldier is a cheap class, and takes little skill. all you need to know is to aim at feet.
Scout you need to be able to shoot while running
spy... it can be hard, it depends on your skill at not being stupid, and sometimes the knowledge of the other team.
Your right about the last 3 though, heavy medic and engy are super easy to play. as is pyro