100 X.P. per hit die is fine, until you consider that there are a lot of special attacks and defenses, differing levels of damage and so on that make that system unworkable. You’ve got to balance your X.P. by the use of breath weapons and magic resistance—and this has always created a problem for me.That '100xp per HD' thing is a reference to the original for OD&D, taken from page 18 of Men & Magic. No doubt more people are familiar with the replacement system introduced in Supplement I: Greyhawk and later adapted for both the Basic and Advanced lines. This later system gives a each monster a base flat amount for the creature's Hit Dice, bonuses for special abilities, and an additional amount per each hit point. A goblin is worth 5xp plus 1 point per each hp it has, for example.
One big advantages of the later system that a troll with the minimum 9 hit points is worth less Xp than one with the max 51hp. The other is that it addresses all of Alexis's concerns about creatures with many special powers end up being rated more than a monster of the same level who is nothing more than a pile of hit dice with a target painted on it. Personally, I reject both concerns and the entire Greyhawk system in favor of the 100xp per hit die rule, for three reasons.
First, and most importantly, 100xp per hit die is brain dead easy. I can do all the math in my head for parties of 6 characters or less and with a jot or two on some paper I can handle more. Also I can hand out monster XP as the session progresses without delaying the game with chart look-ups and such. And by the end of many runs I'm tired and my ability to do harder math becomes somewhat impaired. I hate doing a bunch of calculations at the end of the session, when my cognitive skills have waned.
Secondly, I like the additional monster XPs at low level. Most PCs are less than 25 dead orcs away from level two. Under the Greyhawk system those same PCs each need to kill 166 one hit die menaces. This change allows me to be a stingy bastard with the initial treasure if I want to. Or more importantly, it allows me to distribute the loot unevenly. If the bugbear twins have most of the treasure on dungeon level one but the PCs never get around to their lair, they still have a reasonable shot at making level 2. Or if the big gem is hidden someplace they never look, their hopes for a relatively quick trip to second level aren't completely dashed.
Finally, I would argue that the inherent unfairness of this method rewards smart players. This is where I think Alexis and I most obviously part ways. His method seems to be attempting to achieve one of those "realistic simulation" thingies. I'm running a game about beating up monsters and stealing their treasure. I know lots of players out there absolutely abhor metagaming like this:
Player 1: Let me get this straight. Plusses to hit dice always round up to the next level, so a gnoll and a hobgoblin are both worth 200 xp apiece, right?
Player 2 (flipping through pages): It says here both have the same treasure type.
Player 1: So we earn the same XP for kills and they got the same treasure, but the gnolls on average have 3.5 more hit points each. Forget the gnolls, let's go fight the hobgoblins!
But by my lights that's an example of smart player thinking and I want to reward that sort of thing. Of course this specific example may not work out the way the PCs hope, as in my campaign hobgoblins are much more likely to post guards, set traps outside their lair, fall back to defensive positions and such. But that's one of the hazards of making decisions based only on theoretical precepts and not actual on-the-ground intelligence.
Alexis has another concern about the Greyhawk XP system: big gronks are overrated. A giant with 8 hit dice and two ogres with 4 hit dice are not equivalent encounters. In many circumstances the ogres are more dangerous because they can make more attacks, are harder to backstab and the party can completely gang up on the giant. Alexis's proposed solution is to give gigantic monsters a lot more hit points. I'm not so sure about that. My gut tells me more attacks or higher damage or both would work better. Or leaving the system as is and letting the players 'metagame', picking on lone giants or dinosaurs because it's smart play.