An insightful tidbit relating to game design:
The real strength of Doom's monsters is that they are all weak enough to serve as a single small piece in a million unique tactical puzzles, while dangerous custom monsters only fit in two. One: you can fight them on their own. This is alright once; then it gets boring. Two: you can fight them in a group. Chances are that this group, custom monster included, will be too dangerous for the player to face head on, so the map's design must afford the player some hidey-hole from which he can safely take pot-shots or enough room to circumnavigate not only the monsters, but also the globe. This is also boring.
— Creaphis, t/nc #377
He does have a very good point there. When you're a novice player, the game's "sweet spot" will still be pretty broad; I think most video games will, in their first levels, appeal to most inexperienced players, and hardly anyone will think that the games are too hard *or* too easy. But as players progress, finding the right balance for a game becomes more and more difficult: the sweet spot narrows, and presenting a good challenge (which is different from a tough challenge) becomes more and more of a problem.
For FPS games, making the enemies more difficult is one way of doing it, but it is far too easy to either make the game entirely frustrating due to being essentially unbeatable without a decent dose of luck, or make it too easy by providing the players with *too* many advantages against their overpowered foes. Hidey-holes and lots of open space that allows for circlestrafing are one way of doing this.
Another way of doing this is to make the player more powerful by cranking up their weapons, health etc., but that carries its own problems: if monsters are twice as powerful, and if the player is as well, you have essentially achieved nothing but an artificial inflation where the numbers are larger but the increase is meaningless (just as purchasing power does not go up during monetary inflation just because the numbers get bigger). In reality, you have to make sure that the player is getting comparatively weaker, but this, again, will run into trouble once you get opponents that are overly powerful and can kill you too easily.
Anyhow, long story short: I don't have an answer for how to balance games well, but the point about DOOM managing to do it by using many small monsters that aren't a threat independently is spot on. And there's quite a few DOOM WADs that provide challenging and well-balanced gameplay for veteran players as well.
Case in point: ever wonder why megaspheres (artifacts that set your health and armor to the maximum of 200%/200%, no matter what they were before) didn't exist in DOOM I? The most you got *there* was soulspheres, which provided +100% health (up to a maximum of 199/200%), and megaarmor, which set your armor to 200%, and the former of these at least was not an item you'd usually encounter just lying around in levels.
This is something that appears to be happening a lot in RPGs; pen&paper as well, but mostly video RPGs. Your health goes up; your weapon damage goes up; your foes' health goes up; your foes' damage goes up. In the end, it may well be that nothing changed, and this is particularly visible when the battles don't rely on reaction (so that you can't just make foes quicker to make them more of a threat).