There are many possible ways for a reviewer to write negative, undermining comments without appearing to be too vicious, and therefore retaining a semblance of professorial dignity. One species of undermining comment is along the lines of
This paper is not significant because [insert major results of research] is already well known.
In some cases it is easy to demonstrate that this is certainly not the case by citing recent papers by highly regarded people in highly regarded journal that clearly show that most people do not in fact know this, and in fact the prevailing view may be quite different from what the results of the new research show. The new research may be wrong, in which case the argument for rejection of the hypothesis/paper should be based on the science, not on vague statements about what people may or may not already know. Or, if everyone does know, then cite the papers that demonstrate this.
The fact that in some cases the everyone-already-knows-that criticism can be so easily demonstrated as false makes it all the more amazing that someone would attempt this, without sufficient justification, when recommending rejection of a paper, and all the more disappointing that an editor wasn't able to detect the misinformation. A colleague recently sent me some reviews and a negative editorial decision involving exactly this situation. He wrote a strong and well-documented letter back to the editor, but I don't think he has heard yet whether the editor is willing to seek additional reviews. I have great, albeit possibly misplaced and delusional, faith in the peer-review system, and think that in some cases these situations may be resolved through calm, reasoned communication and determination.
I encountered something similar with one of my own manuscripts in the past year. A reviewer said that another scientist (by chance, a good friend of his) had studied this research topic already and had made all the significant contributions there were to be made, and, if my manuscript were to be revised, that the reviewer's friend, whose work was cited in my manuscript, should be cited "more prominently". In fact, the old buddy of the reviewer missed some rather essential things in his work from a decade or so ago, and my manuscript cited his work respectfully and appropriately (I have no idea how to cite the work "more prominently".. find a way to work a citation into the title of my paper?).
I showed that review to a colleague, and he thinks that, despite my advancing age, I might still be encountering a phenomenon I used to experience much more often and severely of being patronized by a particular group of older scientists in my field. They have had no trouble over the years accepting a younger generation of male scientists into their coterie of respected colleagues, but they have, over the years, occasionally swatted me down with the "We already knew that" remark, even in cases where this is demonstrably false.
In this recent case, the reviewer who said that his old grad school chum had already solved the major issues has put a PhD student on the topic on which I was attempting to publish.
It is very frustrating, but these guys are not likely to change. They are not sexist to the core -- in fact, the aforementioned PhD student is a female student who has been having a good experience with her advisor and who is enjoying her research. That's great. In fact, she wrote to me and said that since our interests overlap, she would like to discuss her research with me and might need to ask me for advice. That's great too, and I will help her if she asks. Maybe I'm a doormat, but I have absolutely no interest in perpetuating the insidious I-already-knew-thatism, even though in this case there is a good chance that I actually already did know that.
2 hours ago