Sandbox gameplay usually means that there is no linear story or path to follow as a player. The game gives players "systems" and "tools" to use, and then the actual "gameplay" or the content itself is generated organically by the players. "Emergent gameplay"

A couple examples off the top of my head are games like:

gary's mod. - The game is essentially just a collection of all the art assets from other valve games (counterstrike, half-life, etc), and a set of tools to use with those assets, to design and build your own fun content.

Minecraft - pure sandbox gameplay. Randomly generated world to explore and some systems for stuff like building and crafting. Thats it. Its up to the player to go out and choose how they want to play.

Eve online - is another example. Throw a bunch of players in massive sci-fi universe, and say "go do whatever you want. Just have fun"

These types of games, and their gameplay is limited only by your imagination.

One very important thing about sandbox games is that, generally speaking, there is no "right" or "wrong" way to play them.

How this stuff would work in wow? I have no idea! Im really excited to find out though! I love sandbox games.