Update: Discussion has moved on to a new thread.
The load more comments links are getting annoying (at least if you're not logged in), so it's time for a new Harry Potter and the Methods of Rationality discussion thread. We're also approaching the traditional 500-comment mark, but I think that hidden comments provide more appropriate joints to carve these threads at. So as of chapter 67, this is the place to share your thoughts about Eliezer Yudkowsky's Harry Potter fanfic.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six. The fanfiction.net author page is the central author-controlled HPMOR clearinghouse with links to the RSS feed, pdf version, TV Tropes pages, fan art, and more, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
I agree that discussing slavery would make perfect sense, given the conversation that preceded it. However, this ignores the conversation that followed it, whose participants seemed to be entirely unaware that they had been discussing any examples of discrimination other than on the basis of sex.
Based on Quirrell's remarks in particular, I'm pretty sure that they'd been discussing rape, in one context or another. As I said in my last comment, I'll provisionally accept that they were discussing it in the context of slavery, since I can't think of any better fit. But it's still not a very good fit.
Another point that I just thought of: Sinistra's "several centuries earlier" should have been simply "a century earlier", for this hypothesis to fit. Several centuries earlier than the early 20th century almost predates modern race-based slavery. (By the way, can we assume that Sinistra's ancestors were enslaved? Her ancestors may well have come from slave-holding British colonies, but are there any likely alternatives?)