Separate profiles have also been done before. I fail to see how keeping all saves in one folder/list, regardless of the number of different games on the go, is a better design than allowing you to keep them separate.
Hellgate London auto-saves when you quit or switch areas (and if you are in the middle of the zone when you quit, loading the game again puts you in a town). Diablo 2 also auto-saved, and IIRC you could not load an earlier save without manually copying/restoring save files.
In D:OS you can save whenever you wish, and make named saves.
The only thing Diablo 2 saved was your character, quest status and inventory. D:OS saves the state of the game world. You can't load an earlier state in Diablo 2; in D:OS you may want to do so to try completing quests in different ways, or save, try out a new spell or talent/attribute choice, and reload if it isn't as effective as you expected or doesn't fit your character build, etc.
