ZDoom

That's sort of what I already implied, as you physically cannot use the image as a line of text.

Could probably use the internal name by default, and if the skill has a nice name then use it instead. Wouldn't help the inconsistency issue, but it'd look nicer than just always using the internal name, printing a blank name for those that use images, or using the image name.

Pretty much. The save info is written as a sort of meta data, so writing the skill string on save wouldn't be a problem in itself. The only limitation comes from not all skills being set as strings but rather images, and thus whatever the string is would be inconsistent.

I suggested the skill number thing ( with the intent of it being displayed as "Skill x", not just randomly tossing the number in the box somewhere ) simply because I imagine the save file already stores the skill that way. Although I suppose it wouldn't be too much effort to also write the skill's name into the save file as well.

Surely, zdoom.exe has access to the names of the currently defined difficulties?

Numbered custom difficulties can have a wide range of meanings, and the numbers are not otherwise presented in the user interface, so games that appear to run with the currently loaded set of wads should probably display the name of the difficulty (with number as fallback). On the other hand; if if this is just a developer feature, and not something valuable to those who play the games, the numbers are more applicable. The numbers also have a more predictable (and short) format.

Then I suppose a number would work fine

I know that DemonSteele, Final Duel (One of my own mods), My other mod (currently secret), Black And Gray, Eriguns 2.0, XWeapon, Satanic Redux, Insanity's Requiem, Tiberium Doom, Crossfire, Hideous Destructor and Russian Overkill have more or less than 5 difficulty levels...
And I'm pretty sure that there's much more...

Demonsteele has 6 difficulties and I'm pretty sure I've seen a wildweasel mod or two with more difficulties. Think Russian Overkill has them, too, although I can't completely remember. They're actually not too uncommon, especially in mods that change gameplay heavily.

Yeah, I suppose the numbers would work...although, how many non-brutal doom mods actually have more (or less) than 5 difficulty levels?

Would it just be the number of the difficulty? Actual names wouldn't work I don't think, considering mods can define their own skills and have more than 5.

There is room for 2 or 3 more lines of text on the right pane, so these same 2 or 3 lines could conceivably be added to the bottom left pane to accommodate both suggestions: If they opt not to add information about the required WADs, the information is still available. If one tries to load a game without the required WADs loaded, a message listing the missing WADs is printed to the console.

This would be a nice thing.
Having the files that the save needs in there would be nice too...

Could the developers add an indicator of the saved game's difficulty in the section of the load game dialog that shows the map name and the time? This would be especially useful for map testing, play-through video recording, and speed running practice.