Well, each class is easy to play and get a few frags in, but to truly master each class takes different skill sets. With scout you need to strafe, dodge, juke, and have marginally good tracking to deal with each other class and put out damage. With pyro, airblasting EVERY projectile has different challenges and rewards, and airblasting people for proper positioning takes skill, as well as knowledge on the secondaries, and how and when to use them. For Soldier, once you learn to fly, you can explode anything you want. For demoman, proper use of stickies is more important than pipe aim, but knowing arcing weapons really helps. For Heavy, a gun, a sandvich, good tracking, and most importantly gamesense are what you need. Engineer is all about strategy, and knowing that you're not chained to your nest. Medics are another easy to play class that to master you need a great gamesense in order to be a supreme use. Sniper is honestly point and click adventure, so quick reflexes and good tracking help there. And spy. Spy is probably one of the most difficult to master classes out there.
tl;dr: Puppies and kittens are adorable