1. Balance
I'm not sure, but I think I saw a Cacodemon in Hell's Arena auto-respawn on Nightmare!. Because the randomised respawn timer can return a value as low as 2.0 seconds this can render "You have to kill everything" levels like Hell's Arena and The Mort completely unwinable unless you're VERY persistant or reasonably lucky.
Or maybe I'm just paranoid and the cacodemon didn't actually respawn. I wasn't paying enough attention.
2. Balance
Oh here's a good one, Thomas thinks Nightmare might be a little bit too easy.
Well, it applys to Arch-Viles too, but the point is that when monsters respawn, they get all their goodies back. Even if you get no XP, it doesn't take a Blade to farm captains and get all the ammo you'll ever need for 10 dungeon levels.
As such, I suggest that respawned monsters should get no items, unless they need them (Mancubi and Revenants don't, for example). If they DO need the items, they should only get half of them back for each respawn (So half, then a quarter, then an eighth...)
3. Doomishness (A word I just made up!)
In Doom I (And Doom II iirc) it was actually possible to get more than 100% kills in Nightmare provided you were skilled enough to kill more monsters than had initially spawned. I believe this is not the case in DoomRL, and I would personally like to be able to get MORE than 100% kills in Nightmare to both keep a better track of exactly how many monsters you killed AND to add sillier messages for skilled players ("He just wasn't satisfied with killing them once." or "This ass-kicking marine killed them all TWICE!" for 200%+)
Of course, revived monsters will still not give XP, just increase your tally.
EDIT: It turns out that the game DOES tally revived monster kills, but it doesn't include them in the percentage (So you might get something like 648 kills out of 1087 = 59% which is obviously off by around 6%)