I think the break-through will be in creating a dynamic world. I feel Horizons was on the right track with creating a dynamic environment (blight, IIRC, it was called and it was meant to change the landscape if the players didn't keep it in check).…
Here are two other ones:
D&D Online: http://www.turbinegames.com/index.php?page_id=64
http://www.gamebanshee.com/dndonline/
Vanguard: http://www.vanguardsoh.com/