"The key, in my view, is to provide multiple, alternate paths to character advancement and development. When someone wants to grind mobs, they can. When someone wants to do quests, they can. When someone wants to defend or assault bases (Tabula Rasa had a brilliant implementation of this), they can. When someone wants to PvP, they can. The holy grail is to create these different ways to advance and develop a character and make them available to people at all times (or nearly all times). The sooner we realize that as developers, the better."