I have to say there were a couple reasons I joined but I'm only going to explain 1 as of now. Basically, I had a thought that is would be so cool if Notch would update and put goblin villages and other friendly NPC villages with their own mines and you could buy property and stuff. That is practically what this is and as soon as I saw it, I was in!
Great idea. A variety of quests would be great such as 'Collect 100 wood blocks for the builder' or even better, have a dungeon set up that is always there. It could also be a way of training for knights?