Anyway, James at The World of Onn suggested a variant that I thought was interesting - you drop the level (or levels) but keep your experience point total. Thus, you have to gain MORE experience to go up in levels than "un-drained" PCs. I like it, actually. A simple annotation on the character sheet would explain how this works (simply something like Level=4* (drained 2 levels)).
Lord Kilgore (among others) suggested that the drain be STR or CON instead of levels.
James at Grognardia asked if there should be a saving throw. I had never thought of it, but I am now.
So, for Daen Ral, Level Drain works like this:
Characters who lose levels (either to traps, undead or extra-planar beings) have their hit points rolled randomly and subtracted from the current total. Spells and abilities lost are also randomly determined (for example, thieves who lose a level lose 50 points randomly subtracted from his/her thieving abilities). An annotation is made on the character sheet that the PCs current level does not reflect the current experience point total.
Levels drained can be regained via restoration spells, wishes or the actions of extraplanar beings.