In the Dresden Files game I'm in, the game master never tells us the target numbers for thaumaturgy spells and skill rolls. We roll, he tells us whether we succeeded, life goes on. I've heard this isn't encouraged for Dresden Files, but this is par for the course for me; my background is largely D&D. Unfortunately, it gets particularly obnoxious around thaumaturgy: the players have absolutely no idea where to stop making declarations and spending Fate points when constructing the spell. We usually stop somewhere around 15 points of power.
The GM's justification is that we're doing something "we don't know enough about," and so we won't know how much power we need until after we've cast a spell.
An example: we were tracking down a warlock with his collection of lieutenants, mana batteries, and other hangers-on, and we learned that he had a total of 13 places that made up a city-spanning sigil. One of his followers, a neophyte wizard we captured and turned to our side, offered to help us find them via scrying (he'd worked out the pattern, but couldn't mark the places on a map without scrying it again). The neophyte wizard was working with gear unfamiliar to him, so the water wizard in our party helped him make the spell. We modeled the whole situation as a straightforward scrying-via-thaumaturgy spell, but the GM wouldn't tell us what the difficulty was. I argued that this was a "partially-known" situation (the neophyte wizard had done this before, but with different foci; he was a new enough wizard that he really needed the training), but he still wouldn't tell us the difficulty, so we just declared various aspects until we came up with 15 power, and that turned out to be enough.
Should the GM be telling us the target numbers for skills and thaumaturgy, or is he right to keep us in the dark? It's slowing down the game a bit as we're going overboard to declare a number of extra aspects to make sure we don't fall short of whatever target number is in his head.
The GM has played more Dresden Files than I have, so he's claiming expertise. Related to the above, we can spend Fate points after the dice have fallen and he's declared whether the action succeeded, but we still don't know what the actual target number is. So if I spend a Fate point on a failure, the action will succeed, but I still don't know by how much. (Nobody's tried yet to turn a miserable roll into a success via massive spending.)