Show topic - Setting difficulty

[cubano]

Here is show topic I'd like to suggest.

Setting difficulties and difficulty driven game design.

Explanation:
In a lot of games from Warhammer and D&D through Genesys and in Fate one of GM responsibilities is setting difficulty for a check/roll/test. Game cannot proceed unless GM sets it. GM already has unlimited power. When she also gets to set difficulty she is in a total control of game, pacing and PCs lives.
What advice do you have on how to set it right and good?

Extension:
In some games GM doesn't have to set difficulty at all - PbtA and Psi-Run to name a few and it fundamentally changes how the game plays, suddenly GM has one less skill to manage and no longer has the ultimate power. How is playing games with difficulty and with no difficulty different and can each of them learn something from the other?