Perhaps the biggest eye opener was how few of the nine available classes are commonly used in competitive play. The most common team has six players one medic, one demo-man, two scouts and two soldiers. The iconic heavy rarely gets a look in. The medic and demo man are considered so powerful that you are limited to only one per team while nobody brings a spies or engineers.
"Imbalance", you cry " The demo-man must be nerfed". Indeed if this were an mmorpg that is almost certainly what would happen if one class was deemed to be so much stronger than others. Players who spend hours developing their chosen class would be incensed to discover that the class was useless at the high level game. In a shooter though it is not such a problem. If one class is too weak for you you can instantly re-roll. Players are happy to figure out what works and go with it.