Taking Out the Trash: Philosophy
One trouble, anyway, seems to be that in philosophy, bad ideas are immortal. I was made painfully aware of this when I took a course in Ancient Philosophy, though I think that I already suspected it. A course in ancient philosophy is about what the ancients thought, of course, so it has considerable historical and anthropological interest in any case, but what bothered me was that we were expected to take, or at least memorize all that stuff at face value. Some of those ancient ideas, like Zeno's paradoxes and the atomic theory of Democritus were of deep and lasting value - we know, today, the resolution of Zeno's paradoxes, and it's a crucial foundation of mathematics - calculus and analysis, and Democritus was spectacularly prescient. Other stuff, like Plato's theory of forms, is mostly an illusion, and Aristotle's Physics is deeply misguided though his Poetics is still profound.
If physics were done that way, students would still waste years studying phlogiston and prime movers.
Probably no subject of philosophy is more important than human nature, and hardly any subject has had more nonsense written about it. I don't want to go quite as far as G.G. Simpson who claimed that anything written on the subject before 1859 was nonsense and ought to be taken out with the trash, but he's close, and the most valuable information on the subject is 140 years more recent than that.