The Load/Save Mechanism has undergone quite a few changes as the series progressed. Going from the simple point save in Diablo I to the sometimes very annoying status save in Diablo II that, though quite innovative, caused more frustration than a sense of achievement.
The save/load mechanism in Diablo I was more simplistic and easier on the players. The players just had to hit the Esc key and click on Save Game to save the state of the game. Upon reloading, the player has all the attributes, items and location that he/she had before the save.
Of course, this elementary mechanism was not a first for any game back then, and many players found a variety of uses for this mechanic. Anytime someone died, they could just reload their last save point. This was also useful if they were on a budget and did not want to buy more Scrolls of Identify or get Deckard Cain to identify your items for 100 gold each. They could use one identify scroll to identify one item in the inventory, reload, equip it if necessary and identify another item.
Since the item lists for Adria and Pepin were randomly generated each time the player entered the confines of Tristram, repeated saving and loading generated a random list whose quality depended on how far the player has progressed in the dungeon. This was especially useful due to Spellbooks being available with Adria and (at higher levels) Elixirs being available with Pepin. This contributed to the rampant hatred towards Elixirs by players that found their deaths at the hands of an elixir-boosted, high-level-spell-casting Sorcerers that became a fad in the heyday of Diablo I Battle.net.
In Diablo II, you cannot save on a whim. Seeing the rampant misuse of the save-load mechanism in Diablo, Blizzard overhauled the system.
The only way you can manually save is if you also exit the game. Pressing the Esc key displays only the Save & Exit option along with the mandatory Options. Conversely, exiting the game forces you to save as well. With the overhaul of the Skill system that did not promote up to fifteen levels of spells, the game did not allow respecializing of Skills, although indirectly which added to the challenge. This also made players depend on luck before doing certain one-time only rewards, like Charsi's Imbue reward and the smashing of Mephisto's Soulstone.
The game also reloaded from the Act's town area, it never saves the location where the game is saved. This feature frustrated players, especially in the jungle regions of Act III, where the Waypoints for the Spider Forest, Great Marsh and the Flayer Jungle are insanely difficult to find.
In spite of these measures, there still exists a rather risky workaround. Therefore, if you were in a really ugly situation and wanted to reload in a safer spot, what you could do is press Ctrl + Alt + Del to open up the Windows Task Manager, shut down the process running Diablo II, and reload in your last saved spot.
In Diablo III, each action performed is automatically saved on the server, therefore it is impossible to lose progress. However, checkpoints system exists for the purpose of dying in battle: if players do not wish to (or can't) resurrect at corpse, they may choose to be revived in town or at the last checkpoint (usually at zone entrance).