I have two minor design, er, guidelines (not rules in any way) that I'm going to discuss. They form a neat little feedback loop that sits nicely with my ingrained desire for symmetry.
The first idea is that repeated failure on the part of the player yields results (in the form of objects or alterations to the gameplay) most likely to help the player to succeed along with a commensurate decrease in difficulty.
The second idea, the other side of the coin, is that phenomenal success yields goodies unattainable by those having gone through the trouble, but also results in a commensurate increase in difficulty.
In other words, there's no reason that achievement and earning can't flow both directions along the power curve.
If we were to chart gamer characteristics with the truly hardcore/skilled on one side and the casual/unskilled on the other, we might get something resembling a bell curve, with the majority somewhere in the middle.
Games seem to do a good job of targeting specific sections of this spectrum. It seems much harder to cover it from end to end.
These two guidelines are not meant to function solely as a dichotomy, some kind of carrot/stick yin and yang. Instead I envision a fuzzy continuum feeding back to the player from different gradations.
By way of illustration I refer to The Simpsons: Hit and Run. I was delighted when this game offered, after five or six failures of the same mission, the opportunity to move on to the next mission anyway. In other words, the game kept track of my ineptitude and tried to minimize frustration by letting me get on with playing the damn game.
I might have been happier, however, if the game also offered to ramp down certain portions of the mission rather than simply skipping it. If the mission involved following another vehicle, maybe lower that vehicle's top speed. If I was told to destroy other vehicles, lower their hit points, or make them cluster together, or follow more constrained routes. And with every handicap given the player, restrict something on the back end. Make this restriction invisible, so that the player might not get the feeling of being denied the full game - and also so that if/when they do go back to those earlier levels and tackle them at the higher difficulties they are delighted to find extra rewards waiting.
How exactly these two guidelines might work depends very much on the specific game. If your aim is a multiplayer RTS, the goal is figuring out how to adjust to individual playstyles so that less-skilled players are given a handicap (but not too large of one) and more experienced players are given goodies that don't increase their advantage (perhaps grant them a unit which is difficult to deploy correctly, requiring a lot of coordination, but confers an advantage on the battlefield).
I really like it when a game rewards, not just an uber-player, but any player. This might be why I gravitate toward platformers, which liberally scatter random goodies throughout worlds but don't demand that all of them must be gathered in order to advance. Even those, at times, can vary wildly in their difficulty from player to player. Jak 2 varied, for me, between light and enjoyable to teeth-grindingly hard; I'm pretty sure I replayed this one particular section at least 30 times in a row.
It would have seemed pretty great if that game had recognized my clumsy-ass fingers and thrown me a friggin' bone.