Yikes! A really old necro thread. I was wondering why everyone was talking about the Scout update until I noticed the dates.
But, to those who are now coming into the game though, I wouldn't suggest playing just one class. It is a team based game and because of that you need a good team composition in order to function well. A team which is made up of four or five Spies is unlikely to play well.
Instead of pigeonholing into a single class or role, get used to playing a number of different team roles. Try out the more offensive classes; Soldier, Demoman, Heavy, and Pyro; as well as support; Medic, Engineer, and some builds of Pyro. Get used to playing the assassin, picking off key players as a Spy, Scout, or Sniper. Or, get used to playing hit and run as a Scout, Spy, or Soldier.
While you're bound to find one class and playstyle which will suit you best, don't get stuck in that role if the team needs something else. After all, this game has team in its name for a reason.