This is a great new take on the MMO concept.
The third paragraph of that article is what really interests me. Just like in any other MMO, players will form a loose community by taking on complimentary roles. But unlike in most MMOs, those roles are not coded templates which players are forced to choose. Players will choose and shape their own roles, often discovering them by happenstance. As a result, there will likely be a lot of room for individuality and enough dynamics to allow many fresh play-sessions (a long-lasting game).
If Cities XL turns out well, I expect MMO developers could learn a lot from it in this regard.