> I see this mostly on reddit with posts simply saying "I agree" or "well said" and if your lucky quoting some interesting part of the article.

That's not what "reaction post" usually means. That's just "reactions" (which I hate just as much as you).

A reaction post is something like "I disagree - here's ten reasons why" or "I basically agree, but here's a different perspective on this particular point".

So a proper blog article, not just one sentence or one paragraph.