When the moonkin was first introduced, people thought nothing of it other than a big iron bird, that shot magical beams of light out of the sky, pummeling its victims into oblivion, and effectively logging their enemies out of the game, banning their accounts with all the moonkin's might. Do you remember that? Of course not, cause it never happened. The Moonkins were a simple feathery bird, nicknamed the OOMkin, that threw out a few attacks, then ran around the area, /danced and watched as it became overly useless around its friends due to its lack of mana and ability to continue attacks.
Although, at endgame in vanilla WoW, it was a formidable opponent, yet still downsized. When The Burning Crusade hit, the class was looked over, tuned up, and changed around. Many people loved the changes, and as a whole, the class took a hit. Its raid effectiveness was damaged to some and buffed for others. The fact that people could use most of the main abilities that were needed in raids, like Abolish Poison, was a slight hindrance in some situations, but not too much to complain severely about. The plus crit and damage that is applied to most of the raid that was introduced in the 2.4 Fury of the Sunwell patch, was a huge help for those who wanted to raid and be useful as well. But, some people were not happy with some of those changes. They still complained about not being completely useful in most situations of PvE and nearly useless in PvP. What this melted down to is not enough sustained DPS in raids, and not enough burst or max damage in PvP, unless you were feral.