Surely intent to deceive has something to do with this. If I set out to use X's ideas in order to solve a problem, and I make it clear that is what I am doing, then that is honouring. If I don't make it clear that is what I am doing, nor could I reasonably expect that all my readers will know this is what I am doing, then that is plagiarism. A slightly different version of your question would be this: "Where does one draw the line between honouring ... and merely rehashing old ideas?" (I love the word 'rehash', by the way, it being literally visceral.) We are probably all tempted by the answer: "a work of philosophy (or art, or whatever) is not a rehash if it exhibits some amount of originality." So, suppose I use X's ideas (and I'm clear about what I am doing) to try to solve a problem that X did not consider, or to write a novel about a kind of situation that X did not. That is surely a sufficient degree of originality to avoid the accusation of rehashing. But originality is not so easy a concept to...

It varies by country: in some traditions, doctoral exams are big public events; in others, small private affairs. At my University, doctoral exams have a chair -- someone from outside the subject who observes, trying to ensure that the process is fair and the regulations followed. Because of this, I have experience of a huge number of doctoral exams, many outside philosophy (let's see, a few from botany, entomology, economics, forensic science, psychiatric nursing, biomechanical sports science, etc.). They are surprisingly similar in the pattern of questioning. Whether they are superior to courtroom cross-examination, however, is moot. This is because the purposes of the two practices are different: a court wants to establish the truth of X (or at least the appearance of the truth of X); a doctoral exam is frankly not interested in the truth of the content of the thesis, but rather in the validity and professionalism of the research and thinking that went into it.

My daughter told me a relevant joke: A university's Dean of Research became frustrated during budget negotiations with the science faculty. "Why do you physicists always need such expensive labs? Why can't you be more like the mathematicians? All they need is paper and a waste basket." After a moment he added: "Or like the philosophers? All they need is the paper." Yes, philosophy is cheap in terms of material resources. It is worth adding that the journals and books tend to be cheaper also, compared to other subjects. However, teaching philosophy is often an intensive, small-group experience -- so that costs. More importantly, like any other subject, the researching philosopher needs TIME. And time is comparatively expensive, with respect to material resources.

I'd like to add a comment to Allen Stairs' excellent answer: it is worth distinguishing between philosophers who write about 'angst', and the experience of angst. In existentialism, for example, the experience of anxiety is often considered to be philosophically interesting (the fact that anxiety is experienced shows something, or even that anxiety itself is a form of showing) but not yet philosophy. Moreover, the philosopher (like everyone else) must spend most of the time in a state of everydayness, false consciousness or whatever, enjoying a Gauloise and an espresso in a cafe in the sun -- or if gloomy, for perfectly ordinary reasons.

How is it that philosophers make their views know to others? By lecturing at a University, for one. But here we have competition for students: Universities with each other, and between departments over students. The student-customer has to make a choice where and what to study. So, in order to be able to lecture, the philosopher must enter into and have some success in competition. Still more so with publishing: since publishers are in competition, the authors must likewise. However, does this competition necessarily influence what philosophers say and think, or just how they market themselves? That is not so clear. Fashion is more difficult still. It certainly often looks like there are philosophical bandwagons that roll rapidly for a few years or decades. No doubt the element of competition is important here: a young PhD student will want to write on a topic that is likely to get him or her a job. But the latest idea in philosophy may be attractive not merely because it is new, or because...