Nothing in my opinion. There is no ownership claim anyways. And also, the answers here are CC licensed (see the bottom of the page) so the link is only missing attribution. I don't know Gonzalo's point of view but most of us already waived even that requirement in Relicensing code from answers (assuming that this re-licensing actually matters). If you are really bothered by this you can place a comment below the page so people can find it back here.
So it is just annoying to not to have the source but I would not go as far as plagiarism although pedanto-technically yes it is. Note that, the whole stackexchange network is duplicated all the time by other websites. Take a popular question search for its text in google and you'll see lots of copy-SE sites.
In general, answering questions here and then claiming ownership of the answers seems a bit contradictory for me. If you are supplying that much of quality in the answer AND want to stay as the owner at the same time, then why are you answering a question on a public domain Q&A site. How is it different than
Look I actually solved your problem, but it's my solution. Ya ain't gettin' it.
Reading a bit more on the list in http://texnorte.blogspot.nl/p/indice-de-post-por-fecha.html we see that the owner mostly skips attribution on TeX-SX answers but the others do have some attribution, thus it might also be the case that the owner thought StackExchange is CC0.