The most important element in PvE is persistence. They integrate a persistenced world with instanced areas which means an amalgam of both instanced and massive environments where instanced events can have domino effects on other parts of the world, or zone. This unique system allow players to meet and interact while exploring while still using instances to further storytelling within the game and for missions and dungeons.
A new Event system will be added into the game to instead conventional quests which are meant to encourage the player to explore the world. In this system, events will happen with or without the presence of the player, and if a player happens to be present at the time of the event, then the player can complete the quest by participating in the event, and the outcome of the event may affect if, or what kind, of event may happen soon thereafter.
Example: You are wandering through the countryside and you see a dragon flying overhead. You and a group try to stave off the dragon. If you are successful the nearby town gives you a treasure.
But if you don't drive off the dragon, the bridge will be destroyed. This will lead to a team of carpenters gathering at the bridge to try to fix it and then you will have to protect them from bandits.
Overall, there will always be something going on in the world designed to allow character of all levels to continuously interact with the game-world.
An NPC companion system will be implemented to help keep the game solo friendly, but characters will be proportionally rewarded if they choose not to use this system. Players will be able to advance their characters to the maximum level without ever joining a group if so desire. Most content will be designed in a solo-friendly way, though often with mechanisms for scaling up in difficulty when more players are involved. This will give players the option to experience the game whichever way they prefer. But the official encourage players to join together to overcome challenges, because of many awesome element won't be displayed without friends.