This post on Dragonsfoot, in a long thread about the subject that was otherwise fairly boring (but long, as it's a subject people love to hate) got me thinking.
Energy Drain no longer drains a level of experience. You keep all your XP, your fighting potential and saves, your spells, your thief skills. You only lose that hit die.
You gain hit dice back only when you level, and for those over name level who have less than 9 hit dice, they continue to gain hit dice until they are at 9, then they get their standard hit points per level.
Makes Energy Drain a bit easier to figure out in the middle of play (no need to recalculate a bunch of stuff, decide which spell slots are lost, figure out how many XP are left, etc.). It also keeps energy drain scary, because every energy drain you take is lowering your total potential hit dice. And you don't want to be level 35 and only have 5 or 6 hit dice, assuming you can get that far...