So good luck with I Wanna Be The Guy at the hardest difficulty level :-P

The first two Thief games are prime examples of this; higher difficulties add new objectives or harder versions of existing objectives and also change areas of the map or open up new areas. This applies to almost every map.

I know at least some of the Metal Gear Solid games change the enemy layouts on different difficulties, granted it isn't really multi-player.
GoldenEye was fun for adding objectives too.

Tactics for hard AI usually rely on taking advantage of the fact that it is pre-programmed and/or some degree of cheesing the game; stuff that doesn't normally fool real opponents.

I play on whatever difficulties the funnest. I'm pretty decent at fps so i usually go for the harder difficulties first but i suck at rpg's and rts so i usually play them on medium or easy.

I played Counter Strike Source on Steam and I actually did a lot better with human opponents than AI opponents. I find people in first person shooter games can be very cocky and sure of themselves always thinking the big machine gun can own the guy with the dual pistols. How wrong they were.
Dual Pistols rule if used intelligently and strategically rather than going in guns blazing. Also you have better agility with dual pistols than heavy machine guns. That poor user on steam was so convinced he would own me with his machine gun. Trouble is with those machine guns is they are rapid fire and you have to reload to often. Kind of leaves you wide open to be shot at with even a noob player with a pistol.

I think Thief the metal age also did the "add more objectives" thing to increase the difficulty. The hardest difficulty needed you to beat the level without killing anyone. It also added other stuff you needed to steal besides the main objective.

My general rules on difficulty:
- I play almost everything at normal difficulty.
- If the game seems too easy, I sometimes bump it up to hardest.
- When the game is Ghouls N Ghosts difficult (insanely so), then I'll lower the difficulty.
- Can't see the full game ending without playing on a certain difficulty? I'll play on that.
- In the case of achievements/trophies, if I can get 2-4+ trophies for beating the game on hardest then I often do. (This sometimes gets you the lower difficulty achieves as well.)
Playing everything on easy too often, may possibly lower your skill level over time, and cause you to not enjoy more difficult games in the future. Yet, I'm not a masochist, and I don't see the need to punish myself too much. I try to keep things balanced. ;)
Playing single player first DOES give you a better foothold in multiplayer. Sure, you're not at the level of veteran muliplay users, but you've had a chance to master the basics of the game before entering the hardcore world of online. All you have to do is adapt to the new strategies employed on the net, instead of struggling with that AND the basics.

I usually always goes for the hardest difficult these days unless it's a strategy game that is not a regular RTS like command and conquer / starcraft otherwise I pick normal. The only game I pretty much play on easy is the X-Com games because even with easiest difficulties my soldiers dies like flies :/

This must be why I always lose online! Its the only reasonable explanation. ;)

Ugh. Don't care for multiplayer, don't have time to 'master' singleplayer these days. I used to throw myself against the hardest difficulty until I succeeded, but I no longer play games for the challenge. I have more fun by just playing the game. So yeah, I'm more of a tourist gamer (like jungletoad) than a hardcore gamer these days, sue me :)
My 'skillz' might have dropped over time, but I'm not competing with anyone anyway.

If you can play Descent on insane, you'll be plenty ready for MP. And if you can play through Descent on insane without losing a life, you'll be one of the best MP players there is! :)

It depends on the game. Many newer games are easier than older ones. Crysis and Crysis Warhead I played on Hard, merely because of the regenerating health. Despite the hundreds (or maybe thousands) of hours I've spent on Counter Strike, my twich reflexes suck, so Hard was quite enough challenge for me.
Batman: Arkham Asylum was challenging on Hard as well, though it gets a lot easier once you figure out how to do certain parts (the in-game hints help too.)
Any strategy game I play on normal because I'm pretty bad at that "thinking" part.
GRID I'm seriously thinking of switching to EASY because I'm constantly wiping out and coming in at last place....

I think the older fellows in here happen to be more on the "tourist gamer" side of the fence, while the younger, more twitchy and competitive crowd are in the "hardcore gamer" side of the fence. Probably has to do with time management, though I think that desire to beat the game on hard and compete and be the best at the game wanes with age. That's my case at least, even though I was never too hardcore with any one game in particular, not even when I was a teen.
Now that I have more games than I have time to enjoy them, my motivation to master any of them is pretty much non existent. As soon as they get boring, they get kicked out of my drive.

I play on whatever difficulty is fun for me. I don't care if I am "the best", I suck at playing MP online competitive games - but I am fine with that. I don't buy games w/ MP in mind, I buy them because the basics of the game interest me.