Abraxas has a very similar approach to monsters as I. He beat me to the punch on writing down a way to generate monster quirks, with my half finished blog post involved mixing and matching from six different tables, and likely never to be finished anyway. I highly encourage you to check out his excellent post. I wanted to contribute, so I made recycled some of my unused tables and tacked them on to the tables found in the link to make tables that use d20s instead of d10s.
Monster Weaknesses (Roll d10)
11 The monster is a terrible boaster, and acts at all times with the most self assured and confident aura. This is just an act, as the monster is secretly concerned that no one remembers its name. It begins any confrontation with a bellowing roar or shout, and much posturing. Were someone to completely ignore it, acting like they saw and heard nothing, the monster would be forced to pretend not to notice intruders in an attempt to save face, and will shrink by a hit dice per person ignoring it, only regaining them after spending an evening talking itself up again in private. It can be shrunk to zero hit dice in this manner, at which point it disappears as though it never was.
12 The monster is a notorious glutton, caring much less for the quality of its meals, than the quantity. It is so greedy that it would even eat food left out in its path, knowing full well its monstrous constitution will protect it from any poison. However, it will gorge itself and become immensely sleepy and lethargic, and retreat to its lair to sleep. In this state, stealth rolls are at a +4. If the monster is awakened, such as by being attacked, it will regurgitate as much of the food as possible (which may constitute a special attack depending on the DM) but will still fight at a -2 due to grogginess.
13 Some monsters owe their state to old fey magic, and as such, suffer from the weakness of those Little people. Weapons made of pure iron bite into its flesh with easy. Roll attacks made using iron weapons as normal, but, instead of rolling damage, count damage die as though they had rolled their maximum amount. The monster almost undoubtedly knows this, and will likely attempt to kill anyone bearing iron first.
14 This monster is cursed so that it may never walk upon sacred ground under any circumstances, and will flee immediately if forced into it for some reason. It innately senses these borders, and will act accordingly. Clever warriors may strike from safety, but should be wary, for there are many sites used by faiths that are not properly sanctified. A clever monster might lull warriors into a sense of safety before revealing that it is not hindered.
15 This monster was once the familiar to a powerful sorcerer, and has grown to terrible proportions and wicked intelligence after decades of eating magical detritus. Deep in its heart, it remembers servitude and bondage. If the monster were to be captured or caged, it would shrink down to its original form and disposition, forever ending the menace it posed.
16 The lights of campfires have long held back the terrible creatures that stalk the marshes at night, and the power of fire is not inconsiderable against this creature. If the creature's lair was somehow flooded with bright lights, it would lose any magical powers or supernatural abilities it possessed until the lights were extinguished. If it was somehow forced into the sunlight, it would, in addition, lose one HD a round until it was reduced to half.
17 The monster considers himself a monster of great peerage, being the descendant of such-and-such great monster or hag or beast. As such, it has nothing but contempt for anyone of a lower station. Conversely, a clever adventurer could potentially present himself as a visiting prince or princess, which would force the monster to sullenly agree to a parley and consider what the royal visitor had to say. As the monster will undoubtedly think it is still the more regal, this encounter will almost unavoidably end in conflict, but a lifetime of blue blood veneration means that the monster must make a Save vs Spell each round to take an action that would directly harm anyone it considers nobility. Should the truly unlikely happen, and the monster become convinced that someone is actually MORE noble than it, it would be forced to sullenly obey such royalty, if only until an opportunity of usurpation (and consumption) presents itself.
18 The monster is tunnel crawler and a cave wriggler. It sneaks and sulks in the dark, moist caves, only emerging on dark, stormy nights when the world is small and miserable. It has lived underground for so long that it has developed a severe case of agoraphobia. While a nightmare to face in its home, it suffers a -4 to all combat rolls if it is forced out into an open space.
19 This monster is a argumentative, difficult, and contrary to a fault. Although it possesses a wicked cunning, it will almost invariably take the opposite stance in any argument, so long as that position is one that it could logically claim. So contrary is the monster, that, while it will not risk its life to do so, it can be stopped in mid battle to have an argument. It will present a succession of straw men, intellectual fallacies, and non sequiters in unending numbers, despite knowing full well it is incorrect.
20 This monster has no weaknesses, and it know this. In fact, it is nearly immune to anything thrown against it. Swords barely chip its hide. Fire only curls its hairs. Spells crackled and fizz against its arcane might. Only something truly destructive, like lava, drowning, or a cave in, could harm the monster. Knowing its own toughness, the monster is unendingly optimistic about its chances of survival, and will blindly charge into any battle without a second thought, and never thinks critically about anything it does in a fight.
Monster Grievances (roll d20)
11 The monster is miserable, and hates anyone enjoying themselves. It is especially incensed by the sounds of laughter and music, which cause it to consider attacking mead halls.
12 A tomb raider buried his loot in or around town for safe keeping. One of the items buried is cursed, and attracts the monster.
13 The monster was forced out of its original home by a powerful shaman, organized militia, or even a larger monster.
14 The monster's mate or child was killed, and it does not know who to punish.
15 Terribly paranoid, the monster kills anyone who wanders outside of town for fear that they are, in fact, assassins come to kill it.
16 Once human, the monster is driven into a fury by anything that reminds itself of its previous life.
17 The monster is devoutly religious, and is enraged by what it views as foreign and duplicitous devils leading people away from the true faith.
18 The monster, terribly ugly, hates anything prettier than it.
19 Someone failed to leave the yearly offering of blessed foods and wine at the temple, and the monster has awoken to punish the village.
20 Roll TWICE. This monster is HELL OF MAD.