I've been talking a lot about the sense of achievement in gaming lately. Why? Because to me, the feeling of achievement tends to come along with fun. Not to say I'll play a game that isn't fun just for the sense of achievement. Music games tend to easily build this in. Konami has plenty of experience with this. Build several fixed tiers of difficulty, and cater for everyone. Newcomers or casual players can easily pick up the easier songs, and those looking for a challenge will keep moving up the difficulty scale - and once they've passed everything, they look to beating their own personal high scores - and maybe to some degree, the world's high scores.
If you look at games of the current age, you realise that nearly every game has difficulty levels to cater for the different groups. I can't say that they're particularly successful, but it does give people a choice - play the game for fun or play the game for a challenge. There are games with difficulty levels reminiscent of ancient times (Devil May Cry and Ninja Gaiden) but even then - to make sure the games reach the most number of customers (meaning the maximum number of people that can play them), there are significantly easier difficulty levels.
Heck, even RPGs are getting difficulty levels - what the hell? It's become quite apparent that games are becoming easier as time passes. I think this is a sign that people are beginning to realise that there are a lot of different gamers who are willing to spend different amounts of time with a game. If you are one of those gamers who always went for the easy mode for a game and then left it alone - perhaps it's a good time to revisit those games and play them on a harder difficulty.
Some game developers work hard to make harder difficulties different - and often, good games behave very differently at higher difficulties. For some games, it's as simple as enemies being more difficult to kill and dealing more damage. For others, it's not as simple. Enemies gain new moves, shorter response times, behave differently - and often some of these changes are massive to gameplay. That rock you once thought to be in the middle of nowhere - is now a life-saving piece of terrain cover.
That enemy you once thought to be easy to take down, can now kill you in one hit - and your tactics for taking it down have to change considerably, since shooting it down is ridiculously ammo-consuming. That weapon you thought to be the antiexistence of any enemy you had - now only works on one of those enemies effectively. Your cover tactics may change from tactical stand behind cover - to run head on into the enemy and dodge every damn thing. The addition of one grunt may have seemed trivial before - but you now realise why that one grunt made things different. Luck becomes significantly more important to you than ever before - and so does skill.
And so, remember that game developers now have to create 3 different games instead of one - so don't complain when there's a delay. :P