It looks like you're new here. If you want to get involved, click one of these buttons!
There really is no true definition of "Sandbox MMO". However, the ultimate idea is geared around self-sustaining content, e.g., player economy, large worlds and versatile progression. There is no set path that will lead you to the end; there is no point A to point B.
The only chance the sandbox genre ever had, if you would call it that, is with Star Wars Galaxies. Its ultimate demise scares the hell out of developers, as if SOE's / LA's failure was because the game was considered a sandbox.
If you take a look at the master list of MMOs, you will find very few games with the above characteristics. There are probably less than 10. Regardless, the top three active Sandbox games, IMO, are as followed: Eve Online, Darkfall, and Mortal Online. None of these, with mild exception to Eve, experienced AAA development and marketing.
It's never been written that a sandbox game must be hardcore, e.g., first-person, full loot pvp, and full of void material (walking for 15 min without performing some sort of action. However, most naysayers depict the genre as is.
What a shame...