Well, we'll have to ignore the gaping plot-induced stupidity on display by practically everyone throughout the entire story, because without it the books would have been quite short. So setting that aside, because I'm sure it's been trampled to death already.
The complete unwillingness for the wizarding world to utilize even basic Muggle technologies and knowledge is absolutely baffling. It's insinuated that they don't need Muggle things because they can substitute them with magic which is "equivalent." This is self-evidently hokum.
These idiots still write with quills, read by candlelight, don't use the Internet, and despite having literal magic at their disposal their communication systems (such as they are) are laughably inferior to common Muggle ones even in the context of the time period in which the story is supposedly set. Come on. Owls?
Magic users demonstrate basically no understanding of science and are all demonstrably the worse off for it, still having a nearly medieval understanding of how the world works, and rely on magic as a crutch to weakly compensate. This even when it's obvious to an outside observer that a basic piece of mundane knowledge or technology would be not only easier and significantly less dangerous than whatever the fuck their homegrown solution is, but also more effective. This is treated in supplementary works by Rowling as if it's a point of pride by wizards and witches who deliberately eschew anything of Muggle origin -- even if this means going to great lengths to shoot themselves in the foot simply to maintain that attitude of aloofness, which only serves to underscore the sheer stupidity apparently heavily ingrained into magical culture.
The fact that neither Harry nor none of the other Muggleborn kids are puzzled by this, nor why they apparently deliberately fail to bring so much as a common yellow #2 pencil with them from the mundane world out of sheer habit makes zero sense. (And yes, this is touched upon in the already recommended Methods of Rationality.)
Magical consumer goods are also seriously customer hostile. Who the fuck thought even half of those things were a desirable marketable product? Is there an evil wizard version of Willy Wonka lurking around someplace? Think of all the pocket change a Muggleborn lad could make by bringing a case of jelly beans with him to school to sell to his classmates where you don't have a one in twenty chance of one of them tasting like earwax. Or chocolates that can't hop away from you when you aren't looking. I mean, for fuck's sake.
And following from the above, everyone is so concerned about the damage to the karma done by the unforgivable spells, or whatever, which is supposedly why nobody goes to all-out war with the Death Eaters. But then no one gets the brain cells together to realize that Voldemort and especially his goons are surely vulnerable to conventional weapons. All anyone has to do is camp in a corner with a shotgun and then call out they-who-must-not-be-named, enticing them to appear to simply get Swiss Cheesed before having clue one what's going on. Maybe Voldy can't be truly killed by any form of physical harm, but the entire premise of the story begins with the observation that he can be put to considerable inconvenience, putting him down for quite some time, and thus buy the protagonists plenty of time to figure out his stupid riddles and find all his horcruxes. Then simply drive over whatever's left of him with a steamroller.
