I'm going to be running a mini campaign (3.5) shortly that is of the horror centric. My players and I are well versed in 3.x and pathfinder. We tried 4E and determined very early on it wasn't the flavor we wanted. Because of our lack of familiarity with 4E monsters, I figured stealing a variety of interesting/creepy powers from them would give the various re-fluffed 3.5 monsters a unique twist. I would greatly appreciate it if those of you who enjoy the 4th edition would provide me a list of monsters to look at with interesting and/or creepy powers.
I would suggest the Chained Cambion (MMIII) - the Mind Shackles power that "psychically chains" two PCs together is (in my view) quite brilliantly designed. The flavour is that the Chained Cambion screams out is anguish to nearby minds. The mechanic is that the PCs take damage if separated from one another - and a save from one only ends the effect on that one. So first, you have two players who have to coordinate their PCs together, which causes frustration and irritation with one another. Then one saves but the other doesn't, meaning that the one who did save can either leave the other one to suffer (causing frustration and irritation) or has to still suck up the effect (causing frustration and irritation). It's one of the most elegant integrations of mechanic and flavour I've encountered.
The same book also has the Pact Hag, which I found hard to run, but has some interesting abilities around charm/domination.
One of my favourite monsters - simple, but again very flavoursome - is the Deathlock Wight, and especially its Horrific Visage (reposted from the MM update pdf, no longer hosted by WotC I believe):
Horrific Visage (fear, psychic) ✦ Recharge 4 5 6
Attack: Close blast 5 (creatures in the blast); +7 vs. Will
Hit: 1d6 + 6 psychic damage, and the wight pushes the target up to 3 squares.
It's a blast, so only effects those on one side ie looking at the wight. And their minds are blasted (psychic damage) and they recoil in horror (push 3 sq) unless somehow immune to fear effects. Best used near pits, ledges etc.
From MMII, there is the Gorechain Devil that has a non-charm dominate power: it grabs you with its chains and treats you as a puppet!
Are these the sorts of things you're looking for?