It's probably easier to quit if its not the first mmorpg the person in question has played... For me personally, it was more difficult than others for two reasons: family and friends that played it, and because for the type of game it was Blizzard really did execute it nearly perfectly...
I would go in spurts of playing wow for a few months, then letting the account go inactive until I became "over-bored" with wow about 2-3 years ago... to the point where I never once even thought it would be fun to fire up again. To the point where I feel bored with most any new level based, non-loot, non-pvp (or consentual pvp only) "theme-park" mmo just in the time it takes to go from beta to retail, without even playing it.
Theres probably a better term for it than my made up "over-bored"...
Still kinda regret getting my dad and brother to move from UO and even FFXI to WoW.