As a person who likes to play single player, I like the fact that you play the game as single-player but occassionally you are thrown into mutli-player (open instances). Unlike other MMO's you need to go out and find someone to help.
Well... some mmo's have been reaaaaaaally ****ing me off. Spending 40 minutes of my evening shouting /zone Looking for Group <insert class / level> to do <insert dungeon / instance / quest> ... was NOT amusing.
The instances allow you to play solo if you want; or in a group... and the amount of NPC scales. So it's all good. But the instances also have a side effect on the community; sometimes shouting for hours to get a group makes you team with the same people every day; and you form friendships with them; then you dont need to shout; everyone know that at 7PM they'll connect and gather to go hunt some place.