Like many modern shooters and indeed like Far Cry before it Crysis uses automatic checkpoints to save a players progress through the game. Ideally you should have a checkpoint at the end of each significant encounter. Sadly the checkpoints in Crysis seem somewhat random. Some follow one another with no fighting at all in between while on at least one occasion I counted five significant encounters between checkpoints.
In fact I had to repeat that particular stretch at least ten times to get through. The more encounters you have between checkpoints the greater the chance of dying and the more frustrated and pissed off the player becomes as they have to play through that section over and over again.
I was about to abandon the game in disgust when a quick glance at the control setup revealed that Crysis actually has quick save. Its bound to F5. Duh...
Crysis's predecessor Far Cry did not have quick save and suffered from the same randomness of checkpoints. I think it may be a consequence of the open nature of Cryteks games that their checkpoints are so bad. In a more linear game like Call of Duty 4 every player follows the same route through the game and you can place checkpoints at obvious places. In Crytek's games there are many routes to an objective and it is much harder to choose where to place the checkpoints. So we are stuck with quicksave.