Take the Law of Conservation of Matter--when Bruce Banner transforms into the Incredible Hulk, the cells of his body are supposed to expand to a massive size as he becomes the giant green monster. The problem is that according to conservation of matter, he'd have the same amount of total mass in his body even as his cells expanded--the Hulk would then weigh the same as Banner himself, though being a lot bigger, his density would have to be significantly lower. If Conservation of Matter were obeyed, the Hulk's physique be a lot more like the Stay Puft Marshmellowman's than like, well, the Hulk...
You can see I take the fundamental distinction between science fiction and fantasy as resting in attempting to obey the known physical laws of the universe. Sci fi to be properly so called tries to obey the laws of physics, while fantasy throws them to the winds. You could create a sort of science fiction Hulk, but you'd have to have propose something similar to a kind of interdimensional shift where matter existing in the (currently hypothetical) extra dimensions of string theory somehow is pulled into the Hulk's body. That would maybe give the story different flavor--perhaps Banner would have to be portrayed as a physicist working at particle accelerator which somehow blows a rift between dimensions--or perhaps into another universe--an expanding universe of rage.
Usually I can accept a superhero story for what it is, though inconsistencies within a story bother me. So when Superman flies around at will, has raybeam eyes, and is vulnerable to very little except kryptonite, I see story elements that essentially hang together as pure superhero fantasy. Superman is almost like a god from a Pagan myth--he is powerful just because he is; physics simply don't apply. Iron Man, on the other hand, is supposedly a creature of real technology, so his scientific contradictions bother me if I allow myself to dwell on them. For example, he flies by jet power from his feet, right? This is an ordinary technology of course, which maybe could really work in a powered suit--but jet power burns a lot of fuel. The F18 carries something like 13,000 pounds of it to be able to fly around. "Where does Tony Stark have room in his suit to carry all the fuel?" I ask myself. To fly for more than a few seconds he'd need at least a couple tons worth... But then I try to forget about that (and other contradictions) and get back to the story.
Batman and Spiderman are a lot closer than the average superhero to falling in line with science fiction proper (though "average superhero" has to be an oxymoron, right?). Both are mere mortals that perform acrobatic feats not unlike what an Olympic gymnast does. The problem is the gymnasts make their incredible leaps and jumps for a few minutes at most at a time and even then you see them trying to hold back serious huffing and puffing while the judges determine their scores. And these, my friends, are the very best human athletes in the world.
The fact is that Batman and Spiderman both routinely perform feats of extreme physical prowess for a longer time than any real human being has ever performed them. We could wash our hands of them for science fiction purposes and simply enjoy the characters as sheer fantasy, but that not necessary. Spiderman, after all, has a pseudoscientific explanation for his physical prowess--being bit by what at first was a radioactive and in later stories a bioengineered spider. The spider changed his metabolism, so he is no longer a normal human, the story goes, which isn't unscientific per se. It's certainly possible in theory to increase the ability of human muscles to process energy and to exceed current output.
But consider this--if Spiderman has super-efficient muscles and a body energy system capable of outdoing the best gymnast in the world, the Law of Conservation of Energy still requires him to put enough energy into his body to fuel his powerful body organs. If Michael Phelps the Olympic swimmer consumes some 12,000 calories a day in training (a lot of this energy goes to keeping his body warm in the pool, by the way), imagine what Spiderman leaping from building to building for hours at a time would need to consume. At least twelve thousand calories a day, right? Maybe much more. Certainly far more than the standard 2,000 calorie a day diet.
For Batman to keep up it wouldn't be enough for him to have hardcore martial arts training and oodles of technological gizmos. He'd also need to bioengineer his body to be able to outperform mere mortals, something Bruce Wayne could easily afford. But like Spiderman, his caloric intake would have to be pretty huge to engage in the epic fights and leaping acrobatics he does every night. To fuel himself, he'd have to do some serious carb loading before going out into the evening--imagine Alfred serving him a two-foot high heap of spaghetti--on a silver platter, naturally.
Batman and Spiderman are not my creations, so of course my notions of physics aren't going to change these characters. But imagine creating a new superhero, similar to them, who has been bioengineered by his own scientific work or others, with enough technological gadgets to enhance his abilities. And then imagine a story condition in which he (or she) is always hungry, continually struggling to find enough time to get down all the calories he or she needs to face the next fight, continually a Snickers bar, a protein power bar, or a banana in hand.
That would add an interesting dimension to a character, wouldn't it? The poor superhero, trying to scrape up enough money for a shopping cart load of pasta every few days? And by the way, it so happens you'd have crafted a genuine science fiction superhero...