Haven't a good number of shooters started to do this already? I'm sure the AI is improved in certain shooters the higher the difficulty. Can't think of them now though... Halo 3?

a)
Not really a problem on PC, but I know that can be annoying. I really hated when multiplatform came on PC with unlocked difficulty levels.

b)
Oblivion adds just about nothing but hp for enemies when you crank up the difficulty. Only harder thing is with harder difficulties that you need more weapons, because your weapon will break per one enemy because you need to hit them thousands of times.

c)
Well, I prefer games that are hard and have no difficulty levels, because theres always some idiots who say its too easy on easy.

d)
Isn't adding more hp/damage for enemies cheap method? :)

Somewhat related article,
http://www.gamesetwatch.com/2010/02/analysis_is_hard_the_new_good.php

@Mummelmann
I hear you, man. I really don't like either 'teleport all your members into one blob to die'. Especially because I usually take the stealth approach.

And unlimited spawning. Uh, oh. Thats one of them most annoying thing when you have limited ammo.

c) I prefer games to be at the perfect difficulty for me when normal difficulty is selected, but I am not that optimistic.  I would rather have a game have selectable difficulty, then for me to have no choice at all and it be too easy (it seems I don't find very many games that I consider too hard).  Also, having a harder difficulty gives me a reason to play a game again.

d) A matter of opinion.  Like I said before, adding hp is annoying, as it just makes fighting more tedious.  Adding damage isn't a bad idea, because it is encouraging you to fight more effeciently and avoid damage. 

Like I said, option 'd' wasn't in my original group of annoyances about how developers increase difficulty when you select a harder mode.  Option 'd' is how developers incrrease the difficulty when they make the game.  The worst 'cheap shots' are challenges that are unclear, based on luck, or brought about by unintentionaly bad design choices (such as the 'teleport all your members into one blob to die'  event).

I think most people have hit on what I was going to say. 'Cheap' difficulty. However, that can actually take on many versions. When playing an action game, this can be with respawning enemies or a scrolling screen with 'cheaply' placed enemies that divebomb into you at 'perfect' moments that are unavoidable, etc. Good examples of this would be Ninja Gaiden on the NES.

But then there's 'cheap' on RPGS, which can also take on a wide variety of forms, such as in turn based RPGS with enemies rising in difficulty to an exponential level in just one area (causing grinding) or one specific boss which is 10x harder than a previous boss for no given reason, etc. All these things don't add any inherent difficulty to the game aside from making you have to level your character more outside of the normal progression of the game.

Further still, some games, both action and RPG, can have just plain 'cheap' bosses that are nearly unavoidable or nearly perfect in their skills. And take dozens of tries and near flawless play, timing and perfect memorization to beat. These are just stupid and add NOTHING to gameplay, add nothing to the players satisfaction except frustration and do nothing but waste time.

Killzone 2 is actually one of the few titles that had difficulties that meant something.. if you played it on the hardest you would get flanked in a second (I have it platted so I can know :))

Try FEAR 1 on hard if you like flanking...Good luck.

AI that goes crazy when you're in the lead....

My friend and I struggled back in the day to play through NBA Hangtime.

I've also heard stories of playing a Total War game on higher difficulties with an AI faction materializing full stacks out of nowhere.

This bullshit, L4D2 does the same fucking thing, Your doing fine on expert/realism in L4D2 until...

you walk into a room with ONE entrance/exit

you clear the room out so you stand in the door way, when suddenly 3 zombies hit you from behind and knock you down.

Your like "WTF I JUST CLEARED THE GOD DAMN ROOM OUT!!!!!!" 

Bullshit Valve fix the god damn game, its not realistic to spawn zombies out of my ass!!!

Zombies in L4D2 spawn in hordes. The AI director never just spawns 2 or 3. You probably missed them when you thought you cleared the room. Besides, if there are a couple running towards you, they should be easy to hear if there are no other distractions.

Expert Realism can be a lot of fun though, even if the AI director is a douche at times.

Most developers resort to cheap tactics to ramp up difficulty. In general the quicker the game needs to be finished/less money for development the more you'll find cheap tactics to make it 'harder' and therefore 'longer'.

Many a good game suffers this flaw.

Uneven difficulty is sometimes excusable depending on the game mechanics. In some cases where a game has lots of different mechanics, its more just chance you find one that seems unusually hard compared to what came before and then comes after. This is just a risk with some games and its tricky for the developer to balance it.

On the other hand where the game mechanics are pretty consistent or simple, there's little excuse for odd, misjudged spikes in difficulty.

And poor use of cutscenes, checkpoints or re-spawning enemies is simply bad level / gameplay design and should be judged harshly.