One of the conventions of D&D that I always found kind of weird and inexplicable is the practice of adding hit points to a creature's Hit Dice (or in very rare cases, subtracting them.) An ogre, for instance, is listed in B/X as having 4+1 HD. What exactly is the purpose of giving it one measly hit point more than the roll of the dice? Meanwhile, the goblin gets 1-1 HD, because...you want it to be a little weaker than the orc? A few creatures get bigger modifiers, but even so, a bonus of 3 hp is pretty trivial to a troll with 6 HD.
Of course, a creature with a plus to its Hit Dice attacks on the next higher line of the combat matrix - in mathematical terms, it gets a +1 bonus to attack. And creatures less than one full HD attack on a line below the 1 HD line - in effect, a -1 penalty to its attack rolls.
This is potentially a much more useful and game-changing application of HD adjustments than simply adding or subtracting a hit point or two from a monster's total, and one that the game's designers sadly failed to fully appreciate and develop.
One thing D&D doesn't do very well is model the classic mismatch between size and coordination. Fiction and real life are full of examples of big, tough people and creatures that are ponderous and awkward on the attack, and fragile speedsters who strike with uncanny precision but can't endure much of a beating themselves. Hit Dice modifiers are a good way to stretch the system so that it can model that type of monster, though. All we need to do is expand the rule a bit, so that instead of a flat +1 jump on the combat matrix for any addition to HD, you give a bonus or penalty equal to the modifier. A creature with 4+3 HD thus attacks as a 7 HD monster, and one with 2-2 HD attacks as less than 1 HD. This method gains you a little freedom from the direct correlation between monster size and toughness and its skill in battle, without having to add another statistic to a creature's stat block.
Say you want a massive, ponderous beast that can take a pounding before it keels over, but is slow and ungainly in its attacks. Give it a high base HD, with a hefty minus - say, 8-4 HD. It still has a good pool of hit points - anywhere between 4 and 60, with an average of 32 - but it attacks with the same probabilities as a much weaker 4 HD monster.
Or perhaps you want a small, nimble creature that slips past an opponent's defenses with lightning speed. You could give it 1/2 +3 HD, for a total of 4 to 7 hp. That's fairly fragile, but the thing attacks with the proficiency of a 3 HD monster (that's a THAC0 of 17 - as good as a 4th level fighter in B/X or BECMI.) You can take it out in one or two hits, but until you do, it's going to carve you up.
In this way, you can design big monsters suitable for low-level parties, and small monsters that can challenge more powerful parties, without having to make them much more fragile or durable than you want.