A few of my (least) favorite things

[feathin]

My greatest fear right now is druj, since they don't show up with telepathy and have nasty spells that my little Rock Lizard (possessed) monk can't resist and 300 HP doesn't go very far against.

[Other Womble]

Nightwalkers, on the other hand, are very annoying. Especially when you have five of them blocking your progress and Sauron and Kronos bombarding you from behind.

Nah, Nightwalkers can't fly (dunna hrassa time) and can't tunnel (stone prison/spider webs time). They combine nasty melee with some nasty spells (nether ball, summon undead), but there are more annoying critters out there. I'd consider them to be less annoying than Nightcrawlers for starters, though that might have something to do with playing style (I'm a huge fan of Morphic Oils of Spider-generated webs).

[Lord Estraven]

It's not so much their power on their own as their ability to block up hallways by just standing there. When you've got four or five of them in front of you and deep uniques pelting you from behind, their inability to satisfyingly kick the bucket in a couple of turns becomes a hazard.

[Phoenix21692]

My vote goes to the Random Number Generators. Once the breeding goes out of control. they get too annoying very quickly, especially when you're trying to pick the items left behind early in the game.

[elibolas]

Floating Eyes. I've lost more characters to Floating Eyes than anything else. You step around a corner next to one and it's time to start a new character.

-- Elibolas

[Conker]

Nazg?l. No matter what precautions you take in terms of resistance/immunity, nothing will save you from the nasty consequences of one errant keystroke turning your warrior from a 13 blow death-dealer wielding the wonderful +35/+35 vorpal blade to a mediocre monster masher with a relatively pitiful +21/+24 vorpal toothpick.

Very, very annoying, especially if your beloved weapon should happen to disintegrate and/or it's something irreplaceable. *sigh* Now to pray for Sting, I suppose.

[danielfishman]

My greatest fear right now is druj, since they don't show up with telepathy and have nasty spells that my little Rock Lizard (possessed) monk can't resist and 300 HP doesn't go very far against.

Druj do show up on telepathy.

[Other Womble]

My current pet hate is Grand Master Mystics. Even if you avoid their summons, you spend every second turn having to quaff potions to avoid being stunned into unconsciousness, and they just love draining sanity.

[budswell]

Floating Eyes. I've lost more characters to Floating Eyes than anything else. You step around a corner next to one and it's time to start a new character.

-- Elibolas

Definitely

[Xandor Tik'Roth]

Starting to hate on the D's... Especially the Great Wyrm variety since they seem to chain summon. Normally, this wouldn't bother me, but when you've got 6 Great Wyrm's of Chaos all breathing at you with no teleportation, things can get ugly.