What I’m wondering is if the level adjustments on a lot of the creatures aren’t too high. I think it’s possible that in many cases the authors either overestimated to be on the safe side, or misjudged the value of monster hit dice, or both.
Take the ettercap – I know they don’t advance by character class so they’re not great player characters, but lets say for sake of argument that it’s not an issue. Five hit dice plus a level adjustment of +4 makes this guy equal to a 9th level character. Is it really worth it?
Monster hit dice, with the exception perhaps of dragon and outsider hit dice, are actually weaker than player character levels because they don’t grant any class abilities. To properly judge the value of monster hit dice, they should be compared to the most similar class levels, and the monster’s abilities compared to the abilities of a similar class. To aid comparison, hit dice can be modified by Constitution bonus.
The ettercap, our example, has somewhat mediocre Aberration hit dice – d8, only one good save (Will), and cleric base attack. We can compare his five hit dice to cleric levels this way. Assuming the -6 Int, -2 Cha and +4 Wis basically cancel each other out, what we can now compare is what’s left – +4 Strength, +6 Dex, +2 Con, +1 natural armor, a good bite and two weak claw attacks, web, poison and a few skill bonuses, plus whatever Aberrations get for skill points. Compare this to what a cleric gets – five levels of cleric spellcasting, two domain abilities, turn undead, armor and shield proficiency, a higher base Fortitude bonus and 2+Int skill points. By comparing how much better the ettercap’s abilities are than the cleric’s, we can begin to work out how high the level adjustment should be.
Next, it must be remembered that one level worth of level adjustment is not just worth one hit die or one level worth of class abilities – it is worth both. This is an easy area to make the mistake of judging one level of class abilities to be worth one level of level adjustment. The abilities gained in exchange for a point of level adjustment must be equal in value to both one hit die (saves, base attack and hit points) and one level worth of class abilities. It’s not too difficult to do this the equate ability score bonuses to the missing saves/base attack/hit points while equating the special abilities (poison and web in this case) to class abilities.
As I mentioned earlier, Constitution plays an interesting role. The ettercap’s +2 Constitution bonus in this case can be taken out of the equation if we count that it’s equivalent to +1 on Fortitude saves and a hit dice increase from d8 to d10. In this case it’s not valuable to do this since no classes have both d10 hit dice and cleric’s base attack, but the option is there.
In summary, remember when judging monster ECLs that most monster hit dice are weaker than class levels, and that level adjustments are more expensive than you might think.