I have never liked them. I also don't like 3.x "Negative" levels solution either, although it is getting closer.
See "Level" to me has always been an abstraction, a short hand way of saying you are of a certain skill and power. But the level itself doesn't mean anything. Loosing them is not even properly scary. Really it isn't.
Play some Call of Cthulhu or Kult where there is no such thing and I can tell you, things are scary enough.
I also don't like them since it has nothing to do with why an undead needs them or even would be able to do this. If you say the undead "feed" on levels I counter with "ok, why do they even bother with 0-level humans?" OR more to the point, how do 0-level humans survive as long as they can/could/need-to-for-plot-reasons?
Take the example of Dracula. Are you saying Lucy, who we can assume was attacked by Dracula multiple times, but let's just say three, was at least 6-level? 6th level in what? Mina and Harker too?
Lets look at this from the perspective of a low-level character. Loosing a level is bad news. How about from a higher level one, well...bad yes, but not as bad.
Look at the example of the Magic-User. At 8th level they can cast 2 4th level spells, but after a vampire attack they...forgot them? Ok so memory loss is not an uncommon thing after an attack, but something that specific? Ok, so maybe you can stretch and justify it there, what about a Cleric. He doesn't memorize spells, he prays for them. Now after an attack from a vampire he is no longer worthy for these magics? Lost me there.
For me, level drain never made much sense at all. If it is an abstraction of "Life Force" then we already have that in something much more precious than a level and it fits better. Ability Scores.
Constitution is listed in the AD&D 1 Player's Handbook as:
"Constitution is a term which encompasses the character’s physique, fitness, health, and resistance. Since constitution affects the character‘s hit dice and chances of surviving such great system shocks as being changed by magic spell or resurrected from the dead, it is of considerable importance to all classes."Instead of levels I say Vampires (and I'll get to other undead) should drain 2 points of Constitution. It fits better. The more blood loss you have the weaker you become. The less likely you will survive a system shock.
Let's say the average Constitution Score for everyone is 10. Some one like Lucy, described as frail and weak might only be an 8 or even a 6. Certainly enough to survive multiple attacks, not more than say oh... three. Mina, who is described as robust and full of life survives more attacks.
The draining attack needs to be tied into the type of undead, how it was made and what it needs. We already have a precedent for it; the AD&D Shadow drains Strength. Vampires, who feed on blood (or was that forgotten in AD&D?) should feed on Constitution. Other undead then fall in line.
The Monsters
Skeletons, Zombies, Ghouls and Ghasts
The various members of the "Walking Dead" typically do not have drain attacks, though some could have fear based ones. Zombies, Ghouls and Ghasts can also number among the "Hungry Dead" where they eat the flesh, living or dead, of others.
Ghosts, Banshees, Poltergeists
These creatures feed on fear for the most part, so causing fear is their primary goal. The Banshee can kill with her scream and the Ghost can prematurely age a victim.
Drain: None.
Restore: Damaging, HP restored as normal, aging can be reversed
Lich
The mere touch of a Lich is a paralyzing cold attack that has less to do with actual temperature and more to do with the chill beyond the grave.
Drain: None. Paralyzing
Restore: Damaging, HP restored as normal
Mummy
The rotting disease of the mummy is well known, but mummies can also attack via a draining attack. This attack, sometimes known as the Mummy's Curse, will drain the victim of Dexerity and Charisma. Worse yet, the curse is applied after only 1 successful attack and the the abilities are drained at the rate of 1 point each per day till one of them reached 0. At this point the victim dies and can not be raised. They will then on the next full moon become a zombie.
Drain: 1 point of Dexterity and Charisma.
Restore: Remove Curse, Heal or Wish only.
Shadows
Shadow already drain Strength. As intangible, incorporeal creatures they lack the physicality needed to interact with the world. Draining Strength then makes them feel more alive, but it is temporary.
Drain: 1 point of Strength.
Restore: Natural healing, 1 point per hour of rest.
Spectres
These frightful ghostly shades are the result of a curse. They are removed from the physical world, but loathe it. Like the Shadow, the Spectre drains away Strength, but it can also drain Wisdom. Strength is lost much the same way and for the same reasons as a Shadow. Wisdom, the center of reason, is drained because it was something the Spectre lacked to get itself cursed in such a way.
Drain: 1 point of Strength or Wisdom
Restore: Magical healing via Restoration or Heal spell.
Vampires
These apex undead predators feed on the life force of their victims. They drain the blood (and thus the Constitution) from their victims. A vampire can choose to drain 2 points of Constitution per attack, but many will toy with their victims, draining only once per night.
Drain: 2 points of Constituion
Restore: Natural healing, 1 point per week of complete bed-rest.
Wights
Most wights were soldiers or fighters in their lives. Some of that keen intelligence lives on in undead flesh. Since their lives were cut short due to their physical prowess, or lack thereof, the wight jealously steals Strength from it's victims. It does this with a cold touch from beyond the grave.
Drain: 1 point of Strength
Restore: Magical healing via Restoration or Heal spell.
Wraith
The Wraith is a more power incorporeal undead than the shadow, but not quite the twisted evil of the Spectre. The wraith attempts to drain any ability score it can, targeting the lowest score and draining it till the victim reaches 0. They do this not so much out of malice, though that is certainly true, but because they need the life force the ability provides.
Drain: 1 point of lowest Ability Score.
Restore: Magical healing via Restoration or Heal spell.
Succubus
The kiss of the succubus is as deadly as it is desirable. Men (and women) who have survived it can barely describe it, but all agree that no mortal embrace can ever compare again. While the succubus can withhold the draining feature of her amorous attack at will, she won't often do so. This is because the attack of the succubus will drain Wisdom as well as Constitution. Any victim that has lost more than 1/3 of their total Wisdom score becomes hopelessly in the thrall of the succubus. Only a Wish or similar magic can save them. Once the succubus has used up her pawn she may opt to drain Constitution as well or only to finish of the hapless mortal and return to her master with their soul.
Drain: 1 point of Wisdom and Constitution, or 2 points of Wisdom or 2 points of Constitution
Restore: Magical healing, till 1/3 is lost then only via a Wish
Guidelines for Abilities Drained
Other undead or other demons may also have a level draining attack, here is how to convert those attacks to Ability Draining and how to choose which ability is the most appropriate.
Strength or Dexterity: Any attack where the creature is incorporeal, lacking a true body or is otherwise reminded of their own past physical life. Ex. Shadows and Wraiths lack true physical forms and Wights were mostly fighters.
Constitution: The basis of life-force. It is used in system survival roles and adds to hitpoints. Any creature that drains life will drain Con.
Wisdom: The basis of willpower and the desire to continue on. Also the "mature" side of our minds. People with less wisdom give in to temptation easier.
Intelligence: The domain of the mind. Anything to do with mental prowess, psychic awareness or magical talent lies here. Ex. Mind Flayers feed on the minds of others, they "eat" Intelligence.
Charisma: The personality of the character. Also tied to willpower. Some faerie creatures will drain the life and personality out of a person by draining their Charisma. Ex. The Leanan Sidhe or Psychic vampires.
There are certainly other ways to do this and we can even argue which ability various undead can drain. But I think ability drain is far superior to level drain.