An interesting thread. I find it strange that no-one has proposed that there might not be a universal definition, as in one that we all or even the majority can agree upon, of art or what an artist is.
My own opinion (which is all it is, and like certain anatomical parts, everyone has one), is that art, like beauty, is in the eye of the beholder. And the eye of the beholder is definitely influenced by many factors such as culture, background, education, expertise, personality and more.
Just because someone labels it as "Art" doesn't mean that I agree, nor that I have to.
For example, much of the "Modern/Abstract Art" scene I just don't get or relate to. To me that is not art, but often just pretentious scribblings of an overinflated ego. But other folks find the work sublime.
The distinction that some posts make between "art" and "Art" (and similarily "artist" and "Artist" is amusing to me. It is the ultimate in personal judgement being presented as if there was "One True Way" or one overriding definition of what art and artists are. Though it might be insightful and even useful to make attempts at the creation of such an all-eccompasing definition, I don't think there is such a thing. It's my opinion that it's a holy grail, and interesting metaphysical concept that has no actual existance.
So the questions "Is it art?" or "Is that person an artist?" are paradoxes (unless the art contains two Mallards, in which case one might argue that it's a pairaducks).
The masses have been conditioned to look to the "experts", such as art critics, gallery owners/curators and the like to tell them what "art" is and which people warrant the term "artist". It is a convenient and lazy approach that most folks are happy to assume, since it's the path of least resistance and doesn't require the engagement of any brain cells.
But I would suggest that this is a disservice to humanity and the arts. The proper questions, in my mind, are "Do I think it's art?" and "Do I think that person is an artist?". And my answers may be very different from yours. These questions also do not presume that you need "like" the piece(s) in question to answer in the affirmative. I have seen lots of art that I didn't especially like, though I did consider it art.
Art can be compared to pornography in this respect, as in "I can't describe it, but I know it when I see it".
Philosophical arguments aside, I have always believed that art typically requires technical mastery of the artist. That does not mean that the art itself need be technically masterful, meaning it displays the artists fluency in intricate detail, though it can be. I'm thinking of the Oriental masters as examples of this, where their technical mastery was unquestioned and the product of decades of intense study, work and apprenticeship, but their "master works" were just a few brush strokes on a canvas. Ah....but the strokes were technically and aesthetically perfect in their conception and execution.
In photography, technical mastery encompases control of exposure, focus, depth of field, and the like. But it also slides into the areas of composition, since there seem to be some common "rules" of composition (ie. rule of thirds, golden mean, etc.) which mastery of could be seen as a "competency" that is to be learned. As with the definition of art itself, competency slides into a grey area somewhere in the realm of composition.
It's been said, to paraphrase, that the true master knows what rules to break and when. The unspoken implication is that they have learned the "rules", meaning they have the ultimate in technical competency. There are also some situations where a display of exquisite technical mastery might be considered art in and of itself, even in the absence of narrative/metaphor, but I think that is rare.
A sunset (the sunset itself, not a depiction of a sunset on a canvas, photographic paper or other medium, is typically not considered art. It is beautiful. It can inspire strong emotions (joy, peace, contentment, etc) in people. It's composition can be perfect. But is it art? Most people will probably say it is not.
Building on that, most definitions of art seem to imply, either explicitly or implicitly, the action/will/intent of a person (the artist). The aforementioned sunset can become art by the application of human intent, ignoring the "pretty sunset" cliches that some might raise as an objection. Use another example if you must, for the example subject is not relevant to the point I'm trying to make.
And if human "intent" is a key ingredient of art, as it seems to be, then the term "accidental art" is an oxymoron.
I could leave my camera on a table and my cat could come along and trip the shutter. The image could be technically perfect (autofocus, matrix metering and the like makes that not unlikely), with wonderful composition and engender strong emotions and reactions on the part of viewers. It might even contain strong narrative and metaphor. But it's an accident.
Is this cat-image art? I'll let you ponder that....
My opinion (remember the common body parts?), is that art should also stand on it's own. What I mean by this is that it shouldn't require a huge "out of band" explanation of what it means, what the narrative/metaphor is nor should the external explanation be required to engender emotional involvement by the viewer. The example of the diagonal lines on the paper is a good one. Without the detailed explanation of the creator's AIDS affiction, the piece has no real impact, barring that of confusion on the part of the viewer ('what is this @)$(*$" and why is it in an "art" gallery?). I suppose you can consider that the "art" is the sum total of the graphs and the explanation, but I find this to be contrived and it bothers my personal sensibilities. Sure it brings out strong emotions, but it seems a bit contrived. And technical mastery is very much lacking on the part of the "artist". It was a novel idea, even an interesting one. But is it art? Maybe to you, which is fine, but not to me.
So I guess what I'm trying to get at in this long diatribe is a very simple concept:
art/Art/artist/Artist = YMMV
That about sums up my NSHO on the topic. ;-)