One of my Christmas presents was a copy of the old PSP game Kingdom Hearts: Birth By Sleep (when will they start making titles that make sense in English?). I immediately set the game to "proud mode", the third out of four difficulty settings, because I have played some of these games before and because the title screen informs the player that the "secret" ending is much easier to unlock at higher difficulties.
The result has been an experience that we don't often get in MMO's - the content starts out tuned significantly above the player's head. However, the power curve is intentionally set in a way where going off to farm up even one or two additional levels will allow the player to overcome a previously tough challenge. My experience with MMO's of late has been that the content starts out at "too easy", but the power curve tends to be flatter - if you actually are stuck on something, one or two levels aren't going to help, and you're probably going to have to come back when it's grey (or with a better dungeon PUG, if that's the problem).
Overall, it's an odd challenge that MMO's face as developers aim a larger portion of leveling content as solo players, but that content continues to be the prerequisite for endgame grouping. It's hard to balance one set of content for two very different demographics, and much easier to accidentally end up with a situation where the solo player feels like Werit "drinking from the XP fire hose" as the devs struggle to find the balance with not forcing the poor aspiring group player to endure every last solo quest in the game. I suppose this is the remaining niche for the single player game, until someone comes up with a better solution.