One of the criticisms of Old School D&D, and all class-based RPGs, really, centers around how well class restrictions make sense. Why can't a thief carry a two-handed weapon, or a magic user use a short-sword or an axe? Will one of those characters really sit there and use nothing but their fists if they find themselves in a room with an orc, a restricted weapon and nothing else?
Another point that dovetails with this is how often monsters can use certain attacks; the classic example is dragons, which can only use their breath weapon every few rounds. That really clashes with my intuitive vision of dragons as not only capable of breathing limitless spouts of fire but as unpredictable in battle, at least from their enemies' points of view. The last thing I want to be worrying about when running a battle with a dragon is having the dragon use it's breath weapon and a player exclaim, "OK, move in; he's got four rounds before he can breath fire again!"
I think both of these objections can be dealt with by characterizing the character or monster in terms of "typically won't" instead of "can't." Sure, just about anyone, including a pasty, thin, physically inept weakling, can use an axe in combat without even basic training, and it really isn't a stretch to think that someone who's trained to use a longsword will be able to use a two-handed sword effectively. An out-of-shape magic user who's spent his whole life indoors studying, though, won't know the finer points of using a bearded axe to get past an enemy's shield, and also won't usually want to lug a weapon that weighs a few pounds around on long trips underground or across the wilderness. A thief, by the same token, will want to carry smaller weapons, as much of her job will involve being agile and not having long lengths of steel poking this way and that as she scrambles up a wall or squeezes through a window. She'll naturally prefer shorter weapons to longer ones. Both classes can use restricted weapons (though, and this is an important point, I don't let classes using restricted weapons do any more damage with them than their class weapons do, nor do I let them pack them with the intention to using them at the outset of an expedition), but choose not to unless there is an emergency. Not being able to have your magic user include a battle axe in his pack isn't so much a matter of physical impossibility, but a matter of what your magic user wants; he doesn't want to carry the axe, even if you want him to.
In much the same way, I imagine dragons (and other monsters with restrictions on how often they can use overwhelming weapons) as able to use their breath weapons as often as they want, but choosing not to. Why? Maybe they want to take stock of the battle and can't keep an eye on all those PCs and henchmen scurrying about when flame is bursting forth from their mouths. Maybe they're terribly arrogant about these things. Maybe there's some factor that makes them not want to use their breath weapon; maybe it's painful, or each use takes a day off their lives or something. The point is that what I'm willing to play as what a dragon will (at least usually) do in combats that I run should be what dragons choose to do, not what they are only able to do. It strips dragons of their magic and terror, in my opinion.
And if I ever catch wind that a player thinks that dragons only breath fire every few rounds, you can bet that the first dragon the party meets will breath fire for the first three or four straight combat rounds.