This is the 'Grandaddy' of MMO's and has many fine points but is really showing it's age now.
The main reason, for me anyway, was the excellent PvP that had been integrated into the game. I remember the first time that another player tried to kill me, my heart was beating so fast as I tried to get away, running this way and that, trying to slow them down so that I could get back to the safety of the town. When they struck that first killing blow, my heart sank and I just had to sit back and take it all in, it was incredible that a computer game had made me feel like this.
The funny thing about this game is that graphically, it did not look the best (there were better games out around that time that looked a thousand times more impressive) and the sound was OK and did the job but it was nothing to write home about.
It was the gameplay. The way that you had to work hard with your character, training him up and sending him out to gather resources and kill monsters. Each point that you gained made you feel elated and inspired you to carry on to the next one. It really was great.
The community helped as well, I made a lot of friends in the game, some of which still play now co-incidentally. This was also the first online game that allowed you to buy houses and castles and I remember the pride I felt when I finally had enough money to buy a land deed and set up home for the first time.
But for me anyway, now it has passed it's sell by date and has been surpassed by a lot of other, better looking and sounding games like EQ2 and WoW.
The community is still strong though and expansions are still being released for it so if you fancy a little trip back into MMO history then it may still hold some interest for you. You never know until you try it.