From that enumeration none seems to apply. There is an image with blurred text, but no bad SEO practice to game a search engine bot.
You're right about the pages still appearing with high ranks on specific queries, but on the other hand I attribute it more to Google bombing than page relevance. At least I'm pretty sure Googlebot finds nothing to read on Quora, so the effect must be due to incoming links.
Since Googlebot will dutifully obey canonical, we now have link-bait to the original question with obscured content instead of the non-obscured answers. Again, this is gaming the system. You may disagree, and Quora may as well, regardless that's what's happening.
Edit: Link to webmaster guidelines on rel="canonical" :
Adding this link and attribute lets site owners identify sets of identical
content and suggest to Google: "Of all these pages with identical content,
this page is the most useful. Please prioritize it in search results."
Quora is gaming the system. Period. I won't pursue this point any further.
You're right about the pages still appearing with high ranks on specific queries, but on the other hand I attribute it more to Google bombing than page relevance. At least I'm pretty sure Googlebot finds nothing to read on Quora, so the effect must be due to incoming links.
http://en.wikipedia.org/wiki/Google_bomb