ExploderWizard
Hero
I also don't really buy that there are players who totally don't want to be challenged. I do buy that were are groups where the players don't like to see TPKs frequently and are disinterested in scenarios where their PCs have a high chance of dying, but that doesn't align with hard/easy. You can have something that's very simple and straightforward, and even fair, and yet ultra-lethal and thus no fun to them, or you can have something where the chances of death of PCs is absolute zero, but the chances of failure at what they are doing is extremely high unless they play it very smart (trying to thwart someone who probably can't kill them, but does want to achieve something that they oppose, for example).
There are players who do not want to be challenged, they are only interested in their characters being challenged. It is an important distinction. This is connected to a preference for either gameplay or storytelling as a primary purpose of the game. If death is viewed as a nigh-unacceptable outcome in a narrative then the game has taken a secondary role to the storytelling.
Likewise difficulty can be measured in levels for both the players and their characters. Views on chances of character death can vary widely especially when player input has a great deal more impact on those chances than character capabilities. If smart decisions by the players can mitigate shortcomings in character abilities then more dangerous games have greater appeal. This is where system matters the most. Does success or failure depend largely on player decisions or more heavily on how good the character is in a mathematical sense? In the former case a highly lethal game can be overcome by intelligent play, in the latter case not as much.