WarCry conducted an interview with Chris L'Etoile and John Watson - developers of the upcoming MMORPG title "Ninth Domain".  Here is a snippet:

Warcry: The game's website says you want to mix the "treadmill" games (lots of scripted quests and levelling) with the "sandbox" (players can impact the world and change things) games. How are you planning to do that? How can players impact the world? Chris: Taking care of treadmill players is fairly straightforward. They need things to do and something better than what they have or have done. There always has to be someplace they can go for an hour or two of fun weeknights after work, and they always need to have a "big ticket" weekend adventure to plan and look forward to. They always need to feel that they have more to accomplish and new vistas to see. That implies more than quest experiences; it also means character development (skills) and personal wealth. There's always a better . After you get a horse, you shoot for a house. After a house, a shop. That sort of thing.

You can read the full interview at this link.