If you like music, you really ought to be listening to the podcast Sodajerker. I've been catching up on old episodes, and yesterday listened to the one featuring Jimmy Webb. It's well worth hearing. Not only is Jimmy completely charming and compelling to listen to (and inclined to ramble very amusingly), he also reveals himself to be a fan of Robert Heinlein ("The Moon Is a Harsh Mistress" - I guess I'm pretty thick not to have ever picked up on that) and SF in general.
He also very interestingly and eloquently discusses the importance of surprising the listener in a song - what he calls "benign dissonance". You have to meet the listener's expectations to a certain degree - atonality is a dead end. But without a little bit of dissonance, without at the same time defying his expectations also, you're on a hiding to nothing. You produce very dull and repetitive work. I agree with this. The example the interviewer uses is instructive: in the song "Up, Up and Away", which I'm sure you're all familiar with, there's a very obvious point at which an inferior songwriter would have written "Up, up and away in my beautiful balloon..." and the song would have been adequate. But Jimmy throws your ear off by including an extra "beautiful" in there, so it goes "Up, up and away in my beautiful...my beautiful... balloon". This, to my ear (nobody mentions this in the podcast) gives the song a strange tinge of melancholy that it wouldn't otherwise have. A sort of dreamy wistfulness.
Anyway, this concept of "benign dissonance" got me thinking about doing the same things in games - striking the balance between dissonance and being also pleasing is difficult, but important.
Think about an encounter with a medusa. A bog-standard medusa is just that: bog-standard. The players will know what it is as soon as they see the statues and the woman with snakes for hair. A good DM will make a good encounter anyway, but the potential for an "Oh, it's just a medusa" reaction from the players is obvious.
Now think about an encounter with a medusa which is too dissonant. You can have fun trying to think up your own example, but let's say that instead of turning people to stone, she turns them into carrots. Okay, it surprises the expectations of the players but it is the non-musical equivalent of atonality. It's not interesting.
But approaching a medusa encounter from a "benign dissonance" perspective results in productive outcomes. The medusa wants to be "cured". The medusa has a husband who always goes blindfold. The medusa doesn't turn people to stone - she turns them into amethysts which explode when touched. The medusa is just a child. And so on. You have to turn the dissonance knob to a 5, not to an 11.
It should go without saying that practice and intuition play a big role in how dissonant you should get with your benign DMing dissonance. But it is a persuasive way to conceptualise what it is that makes RPG material good, rather than mediocre.