According to the Savage Species book, monstrous hit dice are equivalent to levels and a level adjustment is than added on top of this to take into account special abilities. IMO, hit dice should not be equivalent to levels. First of all, not all hit dice are equal. For example, dragon hit dice are better than humanoid hit dice. Second, even the best hit dice (such as dragon ones) are not worth an entire level - one must consider that fact that levels grant both hit dice and special abilities - for example a barbarian gains the same 12 sided hit dice as a dragon, but the levels also give him/her rage and a host of other abilities. So, how do you deal with this issue?