Challenge Accepted: What makes a good challenge?

It’s not the fault of the level, it’s the fault of my own choices.
Good challenges are a little like pornography: when you see them, you know it.
Glib though that may be, the fact is that there’s no single formula that leads to a fair and enjoyable challenge every time. Heck, not too long ago I was talking specifically about challenges that work fine in one place but don’t work at all in another game or setting. So let’s be real and say that at best, you can put together the elements that should make for a good challenge whilst accepting that it might all fall apart under scrutiny.
Still, there are elements that point in the right direction. Perhaps it would be more fair to say that simply putting these things together won’t create a good challenge, but a good challenge will assemble all of these in a way that makes sense. Which brings us back to the same fundamental question in need of an answer. What makes for a good challenge?
Mangling terms

Sometimes, admittedly, it’s not necessary to call a game a clone to get an idea of how it plays.
Remember when “clone” wasn’t a term of scorn when discussing a video game?
When people first started saying thing like “Saints Row is a clone of Grand Theft Auto III,” it was actually conveying useful information. Considering the sheer number of games available and the tendency for a new game to closely emulate previous games with a few changes, “X-clone” can often be more descriptive than a simple genre listing. Sure, both New Super Mario Bros. and Sonic the Hedgehog 4 are side-scrolling platformers, but saying that a game is a clone of New Super Mario Bros. provides far more relevant information about how the game plays.
Not that it matters any more, because if you call something a clone of another game, the implication is that it’s a bad game. Because calling things clones has fallen victim to an odd part of discussing games, where we as a culture somehow manage to create and then destroy the terminology we would use to discuss this stuff. It happens everywhere given time, but when it comes to game our new terminology seems to have a half-life of ten minutes before it becomes totally useless.
Endings are the part of a story that tends to get the most press as being complicated, and with good cause. A bad ending makes you wonder why you wasted the time necessary to get to the ending, after all. It’s as true with roleplaying as anywhere else, which is why I’ve had more than a few columns on making satisfying endings in a medium of ongoing roleplaying where nothing ever really ends so much as it sort of concludes.