Oh hai!
Uh, there is no special point like killing Arthas. It's all about exploring the world and training your character. You can do pvp if that's your cup o' tea, level professions, get drunk in Astrub and RP, whatever suits you. There are dungeons, and even some quests if you look hard enough!
Oh, and it's turn-based, so it's probably something new(Or at least should bring back memories if you ever played the older Final Fantasies!).