Sufficiently Advanced.
The basic problem is that in a setting where anything is possible, the players simply don't have any mental reference points that give them the capacity to 'expect' how the world will behave, and figure out how they should behave.
I had a similar experience when our group played Sufficiently Advanced. It's a little-known system that takes place in the far future where technology is so advanced and widespread that nearly anything is possible. When faced with a problem the party usually freezes for a moment, then somebody says, "Well, can we use <fictional technology I just came up with or heard of once> to get around it?" and the GM either just says yes or has the players make a roll of some sort which almost always succeeds.
The characters (and the world at large) are just so wildly powerful that any sort of "normal" challenge is laughable. It's a real task for a GM to come up with something that the players can't just blow through by applying technology. You need a different sort of challenge altogether. It's good for a one-shot, but the GM can only come up with so many dilemmas and logic challenges before he runs out of ideas.
And the mechanics themselves were wonky. The base mechanic was to roll 1d10 and multiply it by some score. It made it very difficult to estimate the target values for tasks (at least, for me).
Another major mechanic was "twists", which could be used to completely re-write the story. This was something that seemed cool, but nobody wanted to use it because we were all polite players and we didn't want to ruin the GM's carefully planned story, even though the GM kept encouraging us to not forget about our twists. It felt like the mechanic was built as a way for the players to formally say to the GM in-game, "We don't like this story."
But the fluff was awesome. There was a civilization where people always wore masks and changed them for different moods or different jobs. Another civilization lived in religious silence, communicating in sign language, because they believed that if you sat in the void of space and listened you'd hear God. In yet another civilization everyone has a mesh (think neural implant) that tells them how to think and feel - voluntary mind control. Another one was dedicated to replaying history, and citizens would live out their entire lives as samaurai or knights or 30s gangsters, using their technology to mock up these historical eras. One group has digitized themselves and they live entirely within computers. I could go on and on with the fascinating and unique civilizations for this game.
So, to summarize, crunch was iffy, fluff was awesome, but the fluff was also awkward to put into play. It would've made a great setting for a series of books.
