In a comment on the previous level drain post, Brandon brings up the similarities to level drain in v3.5, so I went to the d20 SRD to compare the details to what I suggest here. The penalties work about the same as what I suggested, except that I don't suggest altering hit points or spells memorized (they will take care of themselves.) But here's an important part of the 3.5 version:
Negative levels remain until 24 hours have passed or until they are removed with a spell, such as restoration. If a negative level is not removed before 24 hours have passed, the affected creature must attempt a Fortitude save (DC 10 + ½ draining creature’s racial HD + draining creature’s Cha modifier; the exact DC is given in the creature’s descriptive text). On a success, the negative level goes away with no harm to the creature. On a failure, the negative level goes away, but the creature’s level is also reduced by one. A separate saving throw is required for each negative level.What this means is that 3.5 adds a saving throw; if the save is successful, the energy drain doesn't work like old school level drain at all, but returns to normal quickly, while a failed save means the drain is permanent and you must now recalculate your level-dependent abilities, "undoing" your previous gains. Which, in the considerably more complicated 3.5 version of the game, means a substantial amount of changes to the character sheet.
I've heard several people complain that older editions of the game have "save vs. suck" rolls, which they consider bad. But this is a "save vs. bookkeeping" roll; your character is penalized either way, but depending on a die roll, you, the player, get a headache. And I haven't even addressed the problem of tracking level drain from multiple types of creatures so that you can calculate that marvelous DC for the saving throw.
No wonder some people hate level drain. What I don't understand is why they tout 3.5 as being a good fix.