GMing Style

When you run a game of Fate, be it face-to-face or online, what's your approach on it? Obviously it depends on the genre and the feel of the game, but are there constants that don't change? Do you tell the players the Difficulty Level or do you let them figure it out themselves? And if it's not to much to ask, do you have any stories of you with your group experiencing a moment that makes all the challenges of being the GM all worth it?