I agree with most other answers about the right mixture, a good community and founders with integrity, but to me, there is one more important point: The masterstroke of a user interface. You hardly notice it, and it manages to build a flow that makes writing a joy. It provides extremely little obstacles and doesn't remind you of itself on every click. It's like an old english butler always staying in the background, but always there when you need him.
Much of this is surely down to the markdown editor, but it's many other aspects of the UI as well and how things are handled. Probably the result of many hundreds of individual decisions.
I like the UI so much I actually look forward to writing text in it, and I notice how many other forums / sites / wikis / ticketing systems lack comparable smoothness.
The only exception to the rule is the god-awful human test that always pops up when I edit my answers. Why oh why! I really am for disabling that at least for higher-rep users.