Like the guy above me points out, 99% of MMORPGS are terrible and WoW is no exception.
The game requires no skill and very little strategy. It has terrible graphics and physics. The combat is boring. To be 'good' at the game, you simply need to sit at your pc for literally hundreds of hours grinding away. There is no voiceacting and the content remains generic throughout the game.
It also ended the excellent series of Warcraft RTS games (i.e. Warcraft 1, 2 and 3) that spawned WoW.
It also meant that for the next ten years there was an obsession for game companies to copy the game due to its success at being able to pander to the braindead masses, rather than create something original.
Finally, for the privilege of being able to play this abomination, you must pay £7 a month and buy the game and its expansions.