I think that the line is crossed when a challenge is presented that you're completely unprepared for and therefore cannot retaliate against without the foreknowledge that you'll have when you go back to your last save to try again.

That line is crossed in FPS in which you're chance of success hinges entirely upon your knowledge of exactly where the enemies are. Shogo is one example of an extreme repeat offender here, but there are many like it.

RTS sometimes have a similar problem of sometimes throwing things at you in ways that you could not possibly predict. Supreme Commander Forged Alliance did this to such a degree that I lost interest.

The line is crossed in adventure games where you're required to pick things up or do things for no logical reason at certain points, because if you don't you'll wind up dead later on where you can't do anything about it.

I don't want games that are totally predictable. Nor do I want games where I can pull through with a perfect result without hardly trying. But games that just kill you out of nowhere without warning are the result of poor design, rather than an intelligently plotted challenge.

Trust is definitely part of the equation in terms of having to consult breakthroughs in the first place, I find.

A good adventure game is one where you can trust the game creator to create puzzles that make sense so that you should figure it out within a reasonable time-frame if you apply yourself.

A bad adventure game will introduce some awful puzzles and once you have encountered those, the trust is broken: when you become stuck, you can no longer be sure if you'll figure it out within the next few hours or if you just encountered one of those puzzles from hell that just doesn't make sense.

So what do you do? You consult a walk-through in case it's the later (cause you don't really have an entire day to lose solving a puzzle, unless you're retired).

That's why I find it's very important for adventure games to get all their puzzles right.

I pretty echo the sentiments of what other people have said, but I'd also like to add in the camera controls and escort missions. If I die because of the camera being wonky, fuck that.

On difficulty, I think it comes down to personal taste and what mood I'm in on a given day.

Agreed, which is why I'm not that wild about Teenagent. The game has a great sense of humor, but lousy puzzles that make me think wtf how does THAT combination work.

I think the line gets crossed as well, when there is a situation where you need to get this one item earlier in the game. If you don't have you're screwed. Such as the jet-pack in one of the space quest games.
http://tvtropes.org/pmwiki/pmwiki.php/GuideDangIt/Adventure
It is one thing if the game gives you a hint, otherwise that's just cruel.

Reminds me of that POS PoP:SoT, you had that nasty camera and check point system which ultimately caused me to just give up trying to finish the game.

Ideally a game will get gradually harder with some difficulty spikes from here to there. But BS like that stealth sequence in Return to Castle Wolfenstein makes me hate the developers.

I'm looking forward to playing the rest of the AC games, even if people didn't like them as much as AC2 or AC:B. Anybody claiming AC to be superior to those needs to GTFO.

If I have to GTFO where should I go?

I actually have no opinion on difficulty. Some people like to do easy simply to enjoy the storyline and I won't denigrate 'em for it.

To me almost all games without proper save game system is "downright bullshit-sadistic-and-unfair." Almost...

Honestly it depends on my mood. Sometimes I just want to steamroll something, and sometimes I want to have a challenge. I don't really like "bullshit hard" or "fake hard" though, especially if they're combined with things like unskippable cutscenes or wonky save systems. In general, though, I prefer games that have at least some challenge to them.

Also, there are certain types of hard that I find kinda insulting. The final boss of Avadon, for instance, endlessly spawns sub-bosses that reduce the damage the main boss takes while alive. The actual boss doesn't really do a lot of damage, but you have to kill all the sub-bosses in order to DPS him for 10 seconds or so before he re-summons them. It's incredibly tedious, and it ends up being more a fight against your own patience (or your potion stockpile) than it is an actual true boss fight. In short, it's hard for all the wrong reasons.

nowadays we dont have time to go through a level 50 times just becuase its meant to be challenging,

I hate time limits and escort missions. I really do.

Also i hate adventure games that become unwinnable if you make a mistake. This is an example of stupid game design, in my opinion.

Hard is good as long as the AI doesnt cheat nor the game just throws mobs at you which have their hitpoints ramped up or just heaps of them at you

When I was a kid, with no real responsibilities, I could spend as much time as I wanted mastering difficult video games. Now I'm older, and have a great deal less spare time, and am a great deal less willing to spend the spare time I do have playing the same stupid level fifty times. I'm not totally unwilling to play super hard games, but they have to be something pretty special.

Same here along with artificial challenges and I have never enjoyed stressful moments so both timing and time limits are both rather annoying along with broken controls or intrusive GUI.

You could go look for GR's Smut hut, I kind of miss that...