Fun for the first 6 months, then you wake up.
The first few months of play was intense; we're talking getting home from work and playing 6 hours straight. Needless to say the social life took a back seat and you soon find yourself losing touch with friends (and even girlfriends) as your WoW addiction slowly takes hold. The 'grind' from 1 to 60 is what keeps you going, constantly striving to get to the next level. Yes, essentially all quests are the same at a basic level (take this here, kill 20 of these) but interspersed with instances and crafting, this initial grind doesn't become too unbearable.
The decline in my interest of this game started as soon as I hit 60. I never actually got round to starting a 'high-level' instance because I took a look at the people undertaking them and though 'sheesh.. this is just a game and they're taking it so SERIOUSLY!' just look up the 'Onxyia Wipe' on Google to get an example (albeit extreme) of this. I realised I didn't want to spend 5+ hours in an instance to get some new colourful pixels for my character.
Another factor was the politics that go with a game like this; the constant bickering about class changes, talents, weapons, instances, and of course the eternal debate of monthly fees (i'm sorry, but do people seriously think a game of this scope is ever going to be free??!!). It's quite disturbing to see people getting so worked up (and abusive) about something that - in the grand scheme of things - is massively inconsequential.
In conclusion, WoW is an intense experience if you get hooked and be prepared to spend many weeks, months (and in some cases, years) out of daylight. I think we're beginning to see many people coming out of their WoW addictions two years down the line (as can be seen by the recent reviews) but I have a suspicion once TBC is released, the second wave of players will be hooked.