TERA has many issues with balance. In the PvP world, class-based balance was attempted by adding in normalized Battlegrounds (where everyone is put in the same gear upon entering). While this covers one part, the gear, it does not at all account for skill balance and major power differences between the classes. Along with this, these issues even stretch in to PvE content, where there is a pretty big rift between most of the classes and a noticeable difference in how effective each one is at taking down enemies.
The big issue between the classes is that they have different styles of combat. For Archers, for example, you are primarily firing at your targets. For melee classes like the Warrior and Berserker, you have to be right up on your targets. This is a pretty big difference when we look at the Priest (who has target-based healing skills, as well as target-based cleansing ones) and the Sorcerer, who has target-based attack skills. With how much movement there is through PvP matches (especially being that TERA is more of a skill-based game and utilizes active blocking and dodging), this makes a massive difference in the battles.
Gold is another pretty big imbalance in the game, in that there are not many gold sinks now that the game is free to play. Ever since this move, the amount of gold earned has increased considerably while the amount destroyed (by buying things from the NPC and such) has been greatly diminished. As a result, inflation has hit the game hard, and it continues to keep getting worse. So where is the imbalance here? For newer players just getting to the maximum level, they are essentially poor. While they can earn gold to get up to where others are, it takes a lot of time. Basically the lack of gold sinks has caused an issue where the rich keep getting richer and the poor are lagged behind.