This is not an answer but a longish comment, which moreover is certainly "subjective and argumentative". Reading all the stories given in the 12 answers, I find I can classify them in three categories:
(1) the stories that have no factual basis and are pure myths (e.g. the one about Hilbert rejected by Gordan, or Grothendieck rejected by you-know-who, etc.). I'd like to add Fourier to this category but here I don't know the history well enough to be sure. What is certain is that Cauchy, faced with a contradiction between Fourier's result and a theorem he "proved" (limit of continuous function s is continuous) did not dismiss Fourier, and that others quickly dismissed (rightly) Cauchy's result.
(2) The ones that don't really concern mathematics: Boltzmann, Bolzano (whose work in mathematic become admired as soon as it was known, and was controversial for something else), Giordano Bruno, and even Brouwer, who as a mathematician was respected and even admired by about everybody else, and was only controversial as a philosopher of mathematics - and certainly no more than any other philosopher is controversial.
(3) The few ones that have a factual basis and do concern mathematics. I will restrict my attention to these cases as they are the only ones that really answer the question. Now, I am afraid that in each of these stories, where a romantic genius makes a discovery that is ignored or rejected by the conservative establishment of mathematics, my heart is with that so-called establishment, whom I can accuse of no wrongdoing, even with the benefit of the hindsight. Indeed, in none of these cases has been the "romantic genius" persecuted or even bullied (as was for example Giordanno Bruno, or to a lesser extent Gallileo). We, the mathematician community, have no autodafe (not to speak of bonfire) in our history to apologize for. What happens in all those cases is that there was a genial mathematician whose works suffered of serious shortcomings, and it was those shortcomings, and not the ones of the mathematical community, that made the process of assimilation of these works by the community longer that it could have been.
Let me explain my point by discussing some cases:
Levy
Lévy, are we said, was a great probabilist, but not very rigorous. I agree with this description. Now is its non being rigorous a plus, or a minus? To me the answer is obvious, and I hope everyone here agrees with that. At roughly the same time, Kolmogorov was founding rigorously abstract probabilities on Lebesgue's theory, and this gave his theorems a convincing power that Levi's Lévy's have not. The mathematical community has done actually a pretty good and relatively quick work in making Levi's Lévy's results rigorous and putting them in the mainstream theory.
Cantor is an interesting case. An absolute genius, for sure, with sometimes almost idiotic remarks -- like when he writes to Dedekind that his bijection between the line and plane refutes the basic idea of dimension. Dedekind kindly answers to him that people working in geometry only consider continuous functions. Now it is perfectly normal and healthy that his works in set theory was exposed to such an harsh criticism in his time. There were serious foundations problem in what he was doing. From the important point of view of rigor, he was putting mathematics back to the time of the early calculus, forgetting all the progresses in rigor made in the nineteenth century, and indeed, there were as is now well-known some serious paradoxes hidden in his theory. The harsh criticism against Cantor's work (such as Poincaré's) was the anti-thesis in a dialectical process, where the role of the synthesis was played by lovers of the Cantor's paradise that wouldn't want to lose rigor and admit paradoxes, such as Hilbert, who were forced by those very criticism to developed a far-reaching program of mathematics to active its goal. Now the partial failure of Hilbert's program (Gödel's incompleteness and inconsistency theorems) shows that there really was something rotten in Cantor's paradise, and the indecidability of Cantor's favorite problem (the continuum's hypothesis) gives retrospectively weight to Poincaré's criticism: arguably, Poincaré never asked a question which was letter shown to be undecidable, as the continuum hypothesis and the questions of the gender of angels.
Galois? Well, he has the best possible excuses for having written his genial discoveries in such an unreadable way: he wrote them partly in jail, partly the night before his death, and all before he was 22. Now for the very same reasons the mathematical establishment (the "Academie Académie des Sciences", including people with a very different mind, like Fourier) has good excuses not to understand what he had done immediately. And again, very soon after his death (about 10 years after), his work was exhumed, intensely admired and integrated in the living mathematics (especially by the German school).
PS: please feel free to vote down this unromantic post. My earlier self would probably have done so.
