I really like my good ol' Moldvay Basic D&D. Every once in a while, it's missing a rule I want, and I make it up. Then I feel clever. Sometimes, though, I want to know what everybody else did or does. Like now:
Moldvay, B22, gives a table for determining monster experience point value. You find the base value for the monster's HD along with a per-special-ability bonus. Then the answer is simple:
XP Value = BaseXP + (#Abilities * AbilityBonus)
No definition of "special ability" is provided, but monster blocks include an asterisk for each special ability to be counted. For example, the carrion crawler (B32) has a given HD of "3 + 1 *" implying that its total XP value should be 75: 50 + 25, for one special ability. Presumably its special ability is its ability to paralyze.
Mentzer's Basic (at least the set that I have) lists the computed XP values, and it agrees: 75 XP.
In Labyrinth Lord, the carcass scavenger (p67) is a monster basically identical to the carrion crawler. The LL rules don't give per-special-ability asterisks, but do precompute XP. It's worth 135 XP, which would be
65 + 2*35, implying two special abilities, using the XP table on p49.
My copy of the AD&D 1E Monster Manual doesn't give a special ability count or a computed XP.
The reason I find the count of special abilities interesting, especially in this case, is that the carrion crawler has a brutal eight attacks per round. Then again, they don't do damage, they paralyze. Then again, once the party is paralyzed, the crawler will eat them whole.
How are special abilities generally counted? Do extra attacks count? Surely an orc with eight attacks is vastly more difficult to defeat than one with four, and so on. Do weaknesses detract from the expected value? (The shoggoth in LL ROCC is a 7 HD 1590 XP monster, implying 3.286 special abilities, if I've done my math right)
I'm interested in a good rule of thumb, and especially interested in documentation of common rules actually used "officially."