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Many of the WoW players enjoyed the vanilla content because it was like a breath of fresh air; new game, new worlds to explore, etc. Also, most were younger back then and that feeling of nostalgia makes one appreciate the older content due to the emotional excitement caused by it.

The content now is actually more polished and deep, but it cannot replace that experience Vanilla players had when the game just started.

World PvP back then was indeed the most fun aspect of the game, as well as the world boss grieving and kiting to capital cities ;)

My thought exactly :)