There are always Pro's and Con's of every class and often depends on the very skill of the player. At least I've found skill and gear more pertinent than the class of player during my years of PVPing. You sometimes have a class while leveling that seems more overpowered than the next but hopefully at end game they will be balanced.
Agreed on the original statement that most classes do have a nemesis that can more easily take them down.
Example: Melee versus Ranged. It's hard for melee to kill a ranged if he can never touch the ranged. Like snares, crowd control but also limiting the use of cool down helps in those cases as well.
Currently Playing: Aion - (Kera - Cleric, Rains - Ranger and Calysta - Chanter on Zikel)
Previous MMO's: WOW (Previous Hardcore Raider and PvPer), Warhammer (DOK and WE), Guild Wars, LOTR, AOC and EQ.
Other PC Games: Dragon Origins, Mass Effect, Mass Effect 2, Borderlands