Some of the things that is could include are:
1- Need for sleep. You stats go lower if you don't get enough sleep (at least 5-6 hours a day)
2- Need to eat. Your stats go lower if you don't eat every 6-8 hours.
3- No reading or eating during combat.
4- Potions are not consumed immediately (consuming potion is happening outside of frozen time which happens when dealing with inventory)
5- No crosshair.
6- Quest markers are not placed on objects beyond high level maps. (Need to find a person? there could eb a marker for the city, and for the door of that person's house (regardless of whether he is in or not), but not on the person when you can't see him)
This needs to be optional of course, but I believe a lot of people would use this mode.
I'm aware there are some modding attempts to do some of these, but I'd expect them to hit some limitations. Also official support is always better.