Ayn Rand was the author of the novels The Fountainhead and Atlas Shrugged and the founder of the philosophy of Objectivism. Recently several humorous articles have made fun of adolescent enthusiasm for Rand, and pointed out the real dangers of retaining this enthusiasm into adulthood [see Alan Greenspan]. But critics often miss the fundamental flaw in Objectivism: the attempt to combine essentialism with atheism. Rand claimed her favorite philosophers were Aristotle and Saint Thomas Aquinas, both of whom placed God at the core of their essentialist philosophies. Rand also wanted to be an essentialist, to help her derive axiomatic proofs of her absolutist moral and economic theories. Essentialism (and the related ideas of universals and Platonic forms) was strongly criticized by more recent scientifically-minded philosophers such as Hume, Popper, and Quine. Rand has never gotten much respect in academic philosophy, and I am unable to find an academically respected philosopher who is both an atheist and an essentialist. I suspect the combination just doesn't work.
As implied in the linked articles, this combination of essentialism and atheism is probably also key to Rand's appeal to a very specific type of adolescent: one who has recently left their absolutist religious faith behind.