Blizzard has a hard job. A lot of multiplayer games have very linear combatants, Halo for example: Red Team and Blue Team. When Blue Team loses they cannot blame it on the colour of their armour. Each player has an semi-equal chance to get ahead in the combat by getting power ups or advanced munitions. This is definitively not the case with World of Warcraft, as i'm assured all of you know, and i'm beating a dead horse:
Class balance is hard because of very simple reasons: a) Classes need to be diverse in order to convey a sense of variety and choice within the game.
b) In order for classes to remain diverse they cannot have cut and paste abilities that all classes retain, but with a different name and ability picture stapled to them.
c) Since classes retain incongruent abilities in order to appease a) and b) class balancification cannot be perfect.
The point of this tirade (minus the angry-ness) is that classes are sometimes Over Powered, and that is a good, natural occurrence within a game that is always progressing, or evolving I should say. I guess what I really want to get out is this: YOU need to die sometimes within the game. Without you dying there is no class balance, the game would not be fun because no one is winning, because, in order to win, someone has to die, and that someone is you!
When we play WoW we are one in a crowd, there are no shining stars (well there is, but you probably aren't one), no matter how much the stupid quests tell you: "Oh Palyplayer the Slayer, you amaze me with [insert Strength/Agility/Intellect], you have obviously been the victor of many great battles!," you are one of the millions that get told that. Oh, and to people that read this, you should also read the tl;dr (I hate those) because I wrote things in there that weren't in the above body of text.
tl;dr: In order for games like this to be fun, someone needs to die (in the game). Instead of wining about it, learn to keep to yourself a little more and realize you need to learn greater skill, the players that beat you have probably done so.