This game has very focused classes. While you can end up being good at every class, you really just use 1 at a time. In a 4 person dungeon, the successful combination is always a tank, a healer, and 2 damage dealers. The healer just stays back and casts cure nonstop on the tank, or throws a cure at a damage dealer if they end up drawing aggro. Damage dealers just make sure they target what the tank has targeted and kill. There roles are determined before you start, and you stick with your role throughout.
Also, unlike WKC, you will have a unique level for every class. You may play a class to say level 20, but when you decide to take on another class, you start right back at level 1 in that class. In the end, you don't reach level 50 just once, it's a total of something like 15 times? At any given time you're only working on 1 class. The advantage of working multiple classes is that you can mix some abilities. For example, as a damage dealer you could pick up another class's attack boost ability that your class doesn't have.