One for Sorrow

I agree that for Slughorn, at least, the stakes have never been on the same level as they are for Draco. I see what you're saying -- for Slughorn to discover that Muggleborns really aren't inferior would be more like having been told as a child that worms grow in your stomach if you eat something that fell on the floor, and finding out as a teenager that this isn't really true. Perhaps shocking, but it would affect him very little; whereas for Draco, finding out the same thing about Muggleborns would be like his entire life has shifted. He is an extremist, so such a discovery would probably undermine his ideas of race and supremacy. Yet even so, because he was so much more invested in these ideas, the most I can still see at this stage in canon is a recognition that these ideas have been undermined, which would have a bigger impact on him than it would on Slughorn, but I still see the end result as being about the same: would this recognition make the ideas go away altogether? Would Draco be able to reform (that's what's basically being asked here), to look at a Muggleborn and automatically see an equal? From what I've seen, people who admit that they were wrong and misguided tend to do so rather grudgingly (I mean to themselves), and they rarely beg forgiveness for their ideas, but rather for their crimes -- which Draco can't even do, since he hasn't committed any crimes of a racist nature yet. He joined up with the Death Eaters and got scared, not of their ideology or even of what they were doing because of their ideology, but because Voldemort sent him on a suicide mission to kill the pureblooded Headmaster of Hogwarts. And while it stands to reason that if he was having issues over killing Dumbledore (or anyone), he would probably have the same issues over killing Muggleborns or Muggles, as of yet, his disillusionment with the Death Eaters hasn't been connected with his stance on "Mudbloods", or his possible crisis of faith regarding this stance.

That's what makes Slughorn and Draco's arcs so similar to me, though Draco's more invested in the prejudice than Slughorn probably ever was. Both have simply been led or encouraged to think differently through living examples placed in front of them, challenging their ideology.