We want to see each class to actually have a purpose and feel needed in the party of Tera. Skill is important, but you do not want to see a group of Beserkers and one priest clearing everything. Just keep to the basics of MMOs where a certain class is needed.
Lancer just screams defender to us, necessary for the big bad boss fights. Lets say there is a skill the boss does, an AoE skill that does a nearly one-hit kill on everyone. Time to run right behind the Lancers and pray he's holding the block button. That means they will not be doing big damage(Tera Gold). Just enough to hold aggro.
Beserker are your ultimate back up tank. They are dishing out a good amount of damage with the dps, but more importantly, being on call to off tank any encounter for a bit. Switching Aggro between the Lancer and himself.
Slayer the AoE Melee DPS. Not the best out there on single target damage, but when it comes to taking down grouped up enemies fast he should be one of the best if not the best(Tera online Gold).