I define playability as "am I having fun in the moment?" When I log into the game what is there for me to do? Many MMOs act as grindfests where you are repeating a process to reach a single goal. Is the process that you are repeating fun? Probably for a while, but it will soon become boring. Elder Scrolls has shown its quest system, its open world maps, and its ability to accomplish character goals through different ways. These three systems can take the grind out of leveling in the game. On their own they work well, but it is the balance of the three which allows you to have fun from moment to moment.
If I log in any random night, I should be able to pick any one of these three options and play. If I am in the middle of a quest chain, they I can complete it. Or maybe I just want to explore the world tonight and not follow a quest. That choice for a player is very important and how a game like Elder Scrolls allows for players to make that choice is important to their success and retention.
The third part is accomplishing goals in different ways. A few weeks ago at the press event I was able to wear a pirate disguise to walk around the camp without fighting everything in sight. ESO is pretty difficult by the way, and I actually died a few times when pulling too many NPCs. I like the difficulty because it makes you think. Yet, the disguise made it so easy. That is until I got bored and attacked someone. Elder Scrolls has really presented the "play how you want" mentality. Adding that to an MMO is definitely ESO's biggest challenge.