This is only my third day playing. So most of my opinion on classes. Will be based off of my experience with those classes in other games.
Rogue- Not exactly what I would call a party friendly class. It has the highest damage per second to a single target of any class. Though, I know in Shaiya they royally messed with this class in PVP.
Priest- Battle Clerics and Evo Clerics are awesome. Healers tend to be used alot. The problem with playing a healer. Is that you are reliant on other players. Otherwise, you are at a significant disadvantage in the game. Players don't actually need a Healer in most cases. When they do the Healer is often mistreated.
Warrior- You may be able to pawn somebodies face. But you are useless without a priest. Be ready to raid for the rest of your life.
That's the World of Warcraft Warrior anyway. Most warriors play the role of the damage dealer or tank.