It's funny, because as I've got older my attention span has grown when it comes to things I previously found boring like art, film and literature, but my attention span for gaming has contracted massively, to the point where ANY grind instantly puts me off a game. I used to be able to tolerate it when I played old school RPGs like Pokemon and Diablo, but as soon as I got into WoW I just instantly became averse to it. I made about four characters of different classes: every single one of them I gave up on around level 36, where the grind became really noticeable. MMOs padded out by grind are the most masochistic form of entertainment I can think of, where it's literally a competition to see if you can stave off boredom longer than the other guy to be the first to hit the level cap/get the achievement/gather enough badges or whatever. It has the opposite effect on me, because as soon as I see guys run past me who've submitted to the grind and are now decked out, I feel like switching off the game and doing something worthwhile like learning how to make a really good souffle.
I like grinding occasionally, but doing it for too long makes me want to inflict bodily harm upon myself. I have nightmares about grinding the same Owlkin in Winterspring from level 54 to level 60 on my Warrior back in 2005. It took me almost 2 whole weeks, playing 3-5 hours a day.