I'd have to say that a game that pushes you to master its controls instead of just hobbling through the game is a good difficulty. Here's some random thoughts.

-Most shooters these days don't really become more difficult once you get the controls down. Usually, higher difficulties just means the enemies will have more health as opposed to more enemies to take care of. I think this is a pretty cheap way of increasing a game's difficulty since it usually just forces you to be more patient (you take more damage, so you must hide and magically recover health more often).

-I'd say puzzle games usually have a better difficulty curve since you pretty much have to learn the ins and outs of the game in order to keep up (for example, the flash game Continuity).

-A difficult game doesn't 'break' quickly. Fallout 3 is a fun game, but any difficulty the game provides falls apart after you reach Level 10 or so. This is because after a certain point (that varies for everyone) you go from being a poor scavenger to a death tank with considerably powerful weapon and enough money to accomplish whatever goal you have in mind. Again, you can turn the difficulty up, but that just affects how much damage you and your opponents throw at each other, which personally doesn't suit me.

Basically, I agree with:

V.B.D. wrote:

Bioshock is only difficult at the beginning. The enemies don't scale sufficiently in difficultly to keep up with your upgrades and the 2nd half of the game becomes a cakewalk.

-Probably the toughest game that I recall was Mega Man X3 the first time I played it. I didn't have the controls down very well and I got molested repeatedly by the bosses. Nowadays I can bust through the game without dying, so I would say I agree with Not Sure's idea that with enough practice, the faults become your own.