Alea Iacta Est

For most humanoids, monstrous humanoids and other mortal races the rule of adding hit dice to class level and level adjustment works quite well. However, it is a clear disadvantage for characters who are lumbered with a dozen humanoid hit dice when they could be gaining class abilities, extra skill points or extra hit points.

For PCs and important NPCs I will consider, on a case-by-case basis, allowing a player to reduce the number of racial hit dice he must take and increase his class levels to keep the same ECL. This represents training from an earlier age (most likely to happen when you are only ditching one or two HD after the first), or an outsider being created for a specific purpose and and being more specialised from the outset.

This is most likely for races with very high level adjustments, because in no case should a PC's CR exceed his ECL. Some special attacks and qualities (especially spellcasting ability and other things that are much like class features) might be removed if the character is gaining too much power.

Incidentally, for those who are wondering what difference it makes, let's assume that the table in the improving monsters section of the v3.5 SRD holds for removing HD as well as adding them (which I realise it probably doesn't, but bear with me). Then each outsider or dragon HD turned into a (useful) class level increases CR by ½, each HD of animal, magical beast or monstrous humanoid turned into a class level increases it by two-thirds, and each HD of anything else increases it by ¾. If you are changing it to a class not considered useful for that type of creature (a 'non-associated' class), things are a little different; go and check out the SRD (sorry, but I don't even know which book that stuff is in if you have the new core books; probably Core Book II).

Of course, since monster HD are considered equivalent to class levels for the purposes of ECL, the ECL does not change if you swap class levels in like this. Although I accept that some races should retain at least some racial hit dice (especially if their level adjustment depends on their special abilities being commensurate with their CR), I don't think players should be penalised (too much) for playing non-humans, and making them lug dozens of giant HD around when the rest of the party are depending on their epic feats and fantastic spells is just mean.

Example: Most celestials can cast spells as clerics. If a player wants a celestial cleric and is worried about his turning being poor, I will probably let him treat the character as a cleric, exchanging enough of his HD for cleric levels to make the spellcasting a class feature rather than a racial ability.

Example: The Gloom (from ELH) seems to be a member of a relatively insignificant race whose main interesting feature is the fact that the typical member has the special abilities of a 25th level rogue. I would be inclined to assume that they weren't 25HD monstrous humanoids, but rather 1HD monstrous humanoids who typically have 25 levels of rogue (thereby ignoring the racial HD). This reduces their hit points, increases their skill points, and makes them fit in better with the style of my epic games (which focuses on adversaries with classes rather than ludicrous monsters), as well as allowing players to multiclass them rather than being restricted to rogue until 25th level. They have a couple of racial traits and spell-like abilities that are probably dormant in lower-level characters, but besides that are a reasonable race.