So, if you're curious about the title, and as some of you might've guessed, the Sandbox I'm referring to is Cyrodiil. The game world is basically designed where people level up in a mostly Theme Park environment (each factions zones), and then go in and fight over the Province of Cyrodiil. For those who've played DAoC, you understand this concept very well.
What happens is you basically ride the ride in your normal factions zones while you level up, and THEN once you level up, then you go into the main PvP zone, and fight over Keeps/Castles (and in this game also farms/mines), where you can have a real impact on the world, and what happens there... You and your faction actually have a real impact on what goes on there...
Let's say you hear that one of your faction's keeps is under attack. Well, you can go defend it, or you can go do something else. If you don't defend it, your entire faction can lose that keep, and it's now replaced by the enemy faction's guards, their banners, their players, etc. Whereas if you had defended it, it could've had a totally different outcome.
So PvE in your own factions zones becomes a means to an end. The end, being the ongoing PvP battle for control of Cyrodiil. However, if you prefer to PvE, you can stay in your own factions zones and PvE in peace, and that can be an end in and of itself. So you get the best of both worlds...
On top of that, I would like to note however, that there will be some sandbox elements in the actual PvE zones as well. They said they'll be using phasing to allow different players to seemlessly experience the same areas in different ways... For example, one player might see a town full of werewolves, but when they complete a quest to rid the town of the plague, the werewolves will disappear. And another player who hasn't completed that quest will see the werewolves as still being there.
So it seems like they're really going out of there way to make this into a sandbox MMO as much as they are able, in an industry dominated by overly themepark MMO's (WoW anyone?), I say good job.
Having one huge Sandbox zone for players to play in, as well as various sandbox elements in the themepark zones, I'd say that's as good a job as an MMO can do without getting cheesy and having a world that's constantly different every single time a player turns his head.