actually you caught me out there shambler, I thought what you typed in your last post despite reading what headthump wrote. 

something to threaten the player with. The only threats can really be death or then "starting over". Death is more abstract and stylistic often, and gives the players more excitement.

examples:
1) You have a jumping puzzle and if you drop there's a teleport putting you back to the start of the whole puzzle. If the teleport put you just to the previous jumping position, you could more carelessly try each jump until you succeed. The punishment adds a reward too.

2) There's a big monster and you die if you don't pay attention how to avoid getting hit and you aim badly etc..
Here people are prolly somewhat pissed off if they die and start over multiple times. But if you couldn't die then you would just stand there and hold trigger to the bottom until the beast was dead.

So if there was no possiblity of death, the only interaction method from the world to the player would be delay. Death is also delay in many ways in computer games, but it's more abstract and it works by giving the players the idea that "I must not die" and they get excitement from that. They don't need to think further in every situation.

There are some multiplayer games where death is sometimes preferable, and it kinda sucks.

I remember those old lucasarts adventure games where you couldn't die. They were more of a storytelling, exploration and puzzle kinds of games, and that fit them pretty well. 

maybe everybody's spoiled now but who remembers the old games and people talking at school how they just didn't know if they had enough skill to advance some levels etc... If you couldn't die, what would you do with your skill then?

People play golf. Some say it's hard to hit the ball right. But when you get a good shot, it just feels so rewarding.

I don't mean that singleplayer games should be a hardcore sport but there has to be some challenge and challenge means a chance of failure. There has to be a failure mechanism. Whether it's getting stuck or getting killed. I think in action games the getting stuck part would be more disturbing than getting killed. 

Maybe you are blessed with all the time in the world to waste on ridiculous flames and useless chatter, but I don't. Grow the fuck up. 

or whatever