You know what always floors me? When a student who is a terrible writer has no idea that when s/he plagiarizes, there will be just a wee bit of a disconnect between her/his unbelievably poor writing, and the polished academic language s/he is plagiarizing.
I have in front me an annotated bibliography whose annotations are mostly plagiarized either from abstracts in journal articles, or from the first couple of paragraphs of the articles themselves - I have checked every single one of them. What alerted me was the interweaving of sentences that contain no subject/make no sense/are full of ridiculous spelling errors (s/he has used the word "ethnics" instead of "ethics" a full six times in this piece of work - and I am not sure s/he would be able to tell me what each word means), with fragments like this: "a process of defining moral personhood as a question that is always already culturally bound and shot through with complicated power relations". Uh, yeah. And "ethnics".
What is so astounding about this is the complete lack of self-consciousness it reveals. Students have so little sense of language that they can't distinguish their own writing voices from the ones they are plagiarizing, and realize that the difference will be glaringly, screamingly obvious to their professors. Even though - as in the case of this student - they are students who don't do well at all, generally. This is a literacy problem, I think. It amounts to a kind of illiteracy.