Over on the aimed blows post, I mentioned in passing a simple interpretation of standard level drain. For those who don't want to read that entire post: when a wight or wraith drains a level from a character, lower the level, but not the experience; to regain the lost level, the character has to go on an adventure and earn at least one new experience point. If a character has lost several levels, only one level can be regained per adventure (not necessarily "per session"; a character could go on several short excursions, returning to town after each combat or treasure find, no matter how trivial.)
Brendan commented "... that does mean that level and XP are not ways of expressing the same thing". But wasn't that always true? A dwarf saves at 4 levels higher, according to the original books; a more extreme example would be potions of heroism and superheroism, which give temporary level boosts. Occasionally, a monster or NPC has combat or magic ability at a higher level than would be indicated by the creature's hit dice. So, although D&D tends to stick to a strict marriage of XP and level, there are exceptions, and thus no reason not to embrace it fully. In fact, I make it a rule that experience points can never decrease, only increase; there is a correspondence between your XP and your normal maximum level, but your effective level can be temporarily lower or higher.