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Let's be honest here, a game without polish isn't a game many people want to play, really. If it's got more than five unintentional spelling / grammar mistakes in a quest, I'm likely to get bored and quit. Same goes for bugs and the like.
Without polish, a game won't survive. Even if the game is the deepest, and best, MMO out there, that little coat of polish will make the game shine - it'll make it win.
So, for all the budding game developers out there, here's why WoW is the best on the market.
It's not because it's original, or even brilliant - although it is pretty much close to the latter.
It's because it's polished. Don't copy WoW's game design, that'll get you no where - since everyone who wants a game like WoW is playing WoW. Copy WoW's polish, that'll make your game great.
Side note:- Blizzard are well known for their games, and the polish on their games. Ever wondered why they succeeded so well?