As the title says. What does a sandbox game mean to you?
The closest I have come to the idea of what a true sandbox MMO would look like and function like was a MMO I played a while back called Ryzom. When I think of sandbox MMO this is what I think of. Which is a world where nearly all players could create their own quests and share them with the world. It was all done through instancing and event planning tools. But the game allowed users to create unique quest lines and guild based lore questing.
This to me is true sandbox. Unless a MMO gives you the tools to create your own quests and lore it will never be considered a sandbox MMO. Until I see a MOO with a Halo 3 FORGE based system that lets me create a instanced dungeon or overland quest with mob spawns and quest givers and unique rewards. I will never consider it a sandbox MMO. Instead today's genre of so called sandbox MMO's are more fitting of the title of "museum" MMO's.
Instead of "sandbox" MMO's like Darkfall should be called museum style games where while you can explore and make your own path there is not really much to do. Quests are still instituted and controlled by the game creators and users are forced to use archaic systems like private chat to setup guild events or player events using message boards. It's weak and tired to call anything that is simply an open world a sandbox.
Unless you can shape the world and create your own unique story and share it with everyone. It's still just a boring old "museum" or "theme park" MMORPG.
Here's hoping the gaming industry try's it's hand at opening up the world to more true sandbox MMO's with free and open use by gamers for creating user generated content to build upon the developers created world.