I mean, yeah, and I get what you're saying. I guess, my objection would be that your description is how you'd like it to work: "all it does is give the user an indication of the user's post quality", but, in reality, that's not all it would do, frequently (I think) it would also mark users that people just don't like, or people that have views that many people can't tolerate, and whatnot, completely divorced from the idea of whether or not they're putting thought into their posts. I see this often with merit distribution: a lot of people seem to think they should merit posts that they agree with, and starve posts/posters of merit when they disagree. I expect the same thing would happen with any kind of "mark of shame" thing: people would get them just for posting controversial/annoying/upsetting ideas [1].

Of course, you could argue that the same thing will inevitably happen with the scheme I currently have in mind, but, the difference there is that there's no default component to it, so even if a user has made it onto a very-subscribed-to ignore list, from the point of view of someone that doesn't subscribe to that same ignore list, there'd be no outward sign that those user's posts should be taken less seriously than any other user's.

[1] I mean, that's basically what the trust system seems to often get used for. Honestly, if it were up to me, I'd completely dismantle DefaultTrust. I think it's actually caused more harm to the user base than good.