The UI for DD is usually very good at giving me the info I want quickly and easily. If it isn't displayed by default, I can usually find out by hovering the mouse over the relevant element. I've noticed one hole recently. There's no obvious way to see how much health a monster regenerates per explored tile. This can be highly relevant when playing a Monk, or making use of the APHEELSIK glyph.
Perhaps that info could be displayed when a monster is selected, and when you hover over its health icon? (Making it similar to how that info is displayed for the player.)