All quests should be repeatable. The exp cards gives a big bonus to leveling up in this game. But due to the fact that quest are not repeatable, most people were forced to move up to the next maps to do quest, causing an imbalance level between your current level and the monsters.
The experience receive from current monster is not high as well, but that problem will be solve if grinding quest are repeatable.
Its true that grinding is alot easier as a wizard due to early aoe capability, but grinding is difficult for other class. For example, I spend 3 hours as a wizard class and got to lv 24. I spend the same amount of time on a warrior class and havent even reached first rank change yet.
Increasing exp per monsters may help combatant class level easier, but doesnt help non combatant class at all. This also imbalance aoe classes like wizard or cataphact with other class due to the fact that it is just easier to level with aoe. At the end, you will end up having alot of people choosing aoe class rather than other classes.
Increasing exp per exp card is a good option since everyone (non combatant/combatant) could arquire the same amount of exp through different type of questing, but might take too much time to find a perfect balance.
By allowing repeatable quest, this allow the same result of increasing exp card, but less time consuming to make.
For example, there is one repeatable quest in this game that gives you 2 exp lv 2 cards everytime. But that quest stops being repeatable after reaching lv 15. (this quest allow a fast leveling due to exp card+grinding) In the future, maybe also include non combatant quest and class specific quest that can only be receive through your class specialist.
The actual fun of this game starts after rank 3 or 4, when all the strategic class gets introduced. This game would be more attractive if it is easier to level at the earlier two ranks, without relying on aoe/combatant classes.
NOTE: My opinion is base on my experience as a veteran of (kcbt 1 & 2) and (8 hours icbt tester). <–was only able to play for 8 hours since I got the key late and already had plans for the weekend.