Re-typing a pretty big post... Damn internet!
Thousands of quests, huge gameplay area, with a pretty big variety.
The proffessions are great, and can be very different to any other MMO's.
The Auction House functions very well, as well as the social interaction with players, and there are so many.
The ability to re-play again and again for most people.
The massive choice of what to play as (Race/class). The character customisation is pretty good, with Jewelery, facial expression, colour and unique ones for the other races.
The huge ammount of skills for the classes.
The graphics are pretty amazing on Ultra.
The high end content is brilliant, as well as the low end stuff aswell.
It's constantly changing/being fixed, with patches, and so many expansions really.
The tutorial works very well, so easy to get into.
The battle grounds work really well, so it's fun to just go in those and kill things.
The team work for dungeons help with it, as people won't always just want to grind, or even just do damage (Tanking/Healing).
The GM's are friendly and pretty helpfull, which is always a plus.
The great amount of different items, or different TYPE of items.
It almost always sticks to a very clever lore, which some people like to read.
But most of all, it got lucky. It started the craze with this stuff, and any other MMO after it that resembles it gets labeled as a "Copy".
(Edit: I'm not trying to say it's perfect, it can get boring and repetetive, but hey, some people play it for hours and hours a day, and don't find it bad at all)