I find most video games bore me quickly, but none takes the cake like World of Warcraft. It was fun while I discovered it, but the actual "playing" of the game was so bad I often stopped playing it and decided to work on something instead. Getting to level 70 was a painful grind. Even at level 70 there was so much "work" that the few fun things (which really do show up at level 70) just didn't make up for it.
Why so boring? Because there's no cohesive story. "Kill X of Y" is boring in and of itself, but pad a story on to it and I don't mind trudging through it to advance the story. Instead, you get a one page description of some generic problem (e.g., "My neighbour wants a fur coat, so kill me 10 mountain lions.") only to be followed by an additionally generic followup (e.g., "That's not enough; she wants a hat. Kill 20 birds and bring me feathers.") as well as a meaningless reward. The overarching story is lost in a sea of monotonous grinding, where each step is a painful reminder that accomplishing the next step will be just as meaningless. For the love of god, give me something to look forward to at least!