To keep my interest up for 10+ years would require a game with very small, incremental rewards. This could be a system with hundreds of levels, contacts, magic items or more intangible rewards, like political achievements or social contacts. I am afraid I must admit the micro-reward mechanic of MMOs work for me, and I need to be rewarded and reaffirmed with some regularity to keep my interest up. This is very tricky in a level-based system like DnD.
Other than that, I would need lots of interesting NPCs to interact with, and developing relationships with other player characters.
The world would need to have depth and verisimilitude, with an interesting history that feel like it logically leads to current world events. Preferably, it should be developing, not static. This development can be of any kind; political, magical, religious, technological, commercial. But a world that remains static doesn't feel alive enough to bother with over such a long time.