| "Fun" is not a single concept. There is the "fun" of getting drunk with friends. There is the "fun" of spending a saturday evening doing nothing but watching a favorite DVD, alone or with your significant other while munching on crappy microwave popcorn. This are less exhausting forms of fun. There is also the "fun" of doing something dangerous - say driving a car at 90 mph down a deserted stretch of highway. There is also the "fun" of doing something challenging and different - say scuba-diving for the first time.
MMO's (including EQ) are (were) popular because they combine these different forms of "fun" in one nifty electronic package - there's social fun - going blue with your mates in guildchat. There's favorite DVD fun - running an instance, pvping in AB or etc that you're very familiar with just for the hell of it. There's challenge fun - e.g. high-end raiding of new content.
However, there are also things that are not "fun" in MMO's but are more like "work" - something you have to do, but don't reallly want to - like running MC or endless AV.
If it isn't fun an MMO can't last.
Edited for typos |