I often find myself thinking "man i need to come back to this" when playing. For example a Level 0 teleporter, demon chest, or even the location of a shop with a good item. It'd be nice to be able to make notes either on the map itself or just in an in-game notepad of some sort.
I like this suggestion. If I stop mid-level and haven't been able to loot everything in an area (due to full inventory) then I don't remember WHERE those items were the next time I play (If I even remember they were there at all). Being a completionist... that isn't that much fun.