I think the people who are pointing out balance issues either haven't had enough time with the game yet, or just aren't very good.
It's a little presumptuous that some of you think you have the ability to judge the balance of a game despite it having only been out a week. Especially when the game was designed by very talented developer's who tested game balance for months upon months. I saw threads on other forums like, "Overpowered this" and "Underpowered that" within 24 hours of release for god's sake.
C'mon people.
I don't think it takes a pro player or weeks/months of playing to be able to see how a game works right in front of your eyes and how well or badly each character does, especially when many people seem to be consistently getting very similar results from the same characters/weapons vs. not as good results from certain other characters.
There are many things you can tell almost right away about a game, though I will agree that you have to play for awhile to tell other things.
Even if you played one character long enough to "master" it, I think the majority of players would still get higher scores with Scout or Demoman than with Pyro or Medic as they are configured now in the beta.