The contrast between Newton and Darwin is what I find most fascinating.
Both theories provided a comprehensible mechanism to explain some of the great mysteries of their time.
Both waited a great deal of time before they published. Newton did merely because he had it figured out, was a little bit crazy, and apparently didn't find it all that important. Furthermore, he was worried about protecting his invention of calculus. The ideas themselves had nothing to do with the delay. The legwork had already been done by Copernicus, Kepler, and Galileo. He was merely fleshing in and unifying the details of the system, instead of discovering a radical new view. If anything, Newton's work allowed people to go back into an older style of belief with comfort. The applecart of the heavens had been upset, but it turned out the apples all fell in nice, orderly, logical rows. Newton's theory is inherently appealing to the human mind.
Darwin on the other hand, delayed publishing because of the torture he was undergoing within his own mind. Yes, he saw that his theories would be controversial, but more importantly he was living the controversy. For though Darwin's system of natural selection was elegant and powerful, it was absolutely brutal. It created order, yes, but in a way that was so discordant it rails against the built-in preferences of the human mind. We do not want to see order out of chaos, we want to see chaos come out of order. We want to believe that the universe has a benevolent God, not a thousand blind and contradictory ones who operate in ways absolutely foreign and repulsive to our intellect. We want to believe that we are rational beings, that we are not mere precipitates of a great irrational process that optimized us for things we do not value. As if that's not bad enough, the same process even gave us the contradictory values and very repulsion we have to it.
Darwin's breakthrough forces us to question everything we know, everything we care about, and everything we believe at a level that no scientific theory ever really has. That's why it leaps and bounds above Newton's breakthrough as an idea. Newton's mechanics comes like the light of an angel from above guiding the way. Darwin's natural selection is a monster from the deep telling us we are meaningless in our sleep. Rebuilding the light out of that darkness is a monumental intellectual challenge, and so it is no small wonder that so many people today would rather attack than embrace it.