What type of tasks would GMs see as requiring an unknown Difficulty?
It's not so much players knowing the difficulty as players knowing the outcome of a throw immediately.
They already know their target number. Their target is skill + stat. As soon as they roll, they know the outcome.
Compare two rpg throws:
Player doesn't know target:
Like back in the old D&D days. Player rolled. Added modifiers. Then, looked at the GM. "What happened? Did I hit?"
A good GM wouldn't just answer with a "yes" or a "no". He'd add to the game, making the players live through the moment.
"You swing your longsword with all your might. At the last second, the Orc throws up his club to block. But, his timing is off. Your blade slips under his club, biting deep into the Orc's side. You can feel ribs crack, transmitted up the length of your blade."
Player knows target:
Player rolls, looks at dice. "Hey, I hit! I'm rolling damage."
The first is much, much, much more desireable in an rpg.
NOTE: CT suffers from the player knowing the target, too, but only in combat. And, I would find ways around this, like keeping Range secret, or the target's armor modifier.
Of course, with CT, you've got a lot of other types of throws, too, since it is an open, non-structured, task system. It's easy to put this type of stuff into the game with those other rolls.
ALSO NOTE: That it's much harder to do this with T4. Range isn't an option to keep secret because it defines difficulty. Modifiers typically don't have the same weight in T4 due to the variable dice. (For example, a +1 DM when rolling 2D is a much bigger modifier than a +1 DM when rolling 4D.)
The T4 system is just not a good system to use.