Art is, to an extent, subjective; and taste is not wholly divorced from the discussion of evaluating and defining art. For one thing, how our mind operates to perceive, ingest, and value aesthetic remains unclear, rendering any attempts at this stage to make absolute declarations futile.
Are there objective mathematical equations that can be applied to help define art, yes, or at least probably. And to this extent, this is where I agree, at least mostly, with Allen. However, art involves elements that engage visceral, spiritual, emotional, and other human elements of which, again, we have fully yet to understand (we're getting close, but we are not there yet).
RossJ, you can make your point with as much erudite phrasing and deplorable condescension as you want, but none of this proves that you possess the universal truth. In fact, it only proves, and unequivocally and immutably so, that your quite apt at being pretentious and rude. Still, despite these glaring character deficiencies (ones that I can share too), I'm not completely or automatically dismissing your point of view.
However, the linking of relativism with totalitarianism argument is one commonly used by those in the anti-modernist camp. At best, it's histrionic reasoning, and arguments over art should not be seen as a dangerous politico-ethical gateway to justifying the slitting of an innocent old lady's throat.
Getting back to art and the subjective/objective argument. Again, I do actually believe that when defining "art," there are objective elements, a grammar so to speak, that serve as a guideline, ones that transcend cultural and social differences. I'm not so relativistic as to allow any object (toaster, painting, back scratcher) to be defined as art, simply because one person proclaims such object to be so.
Yet, there is a level where subjectivity and, inevitably, as we are human and not Vulcans, taste come into play, leaving us with a degree of ambiguity. No better proof of this, as Jetcode noted, is that you can get five intellectual art historians/experts in the same room---all of them as self-assured and educated as RossJ---and yet disagreement will ensue, and I'm not simply referring to arguments based on whether one would want the goddamn debated piece on their wall or in their living room.
Now, does such disagreement prove that all is completely subjective, no, and nor should such disagreement overshadow the unmentioned consensus.
My point is that art appreciation, even in its most objective approach, does not course along a single narrative. Art is not a science project, and the injection of humanity will invariably add components of subjectivity into the issue. On the other hand, certain foundations exists that can be highly instructive when trying to approach art, whether as a creator or viewer. This is where I certainly agree with Allen.
As to specifying these guidelines, I cannot. For one thing, I haven't studied art (RossJ saying to himself, 'thanks for pointing out the obvious'). I don't have the parlance down, and I haven't memorized any debate points.
Yet, when I picked up on photography six years back, I quickly started analyzing photos that I found compelling. I often did not know who the photographers were beforehand, so it was not a case of giving credit simply based on name.
I've tried to dissect what components on the visual and emotional side lent to the success of the work. Before photography, I was very much (and still am to a lesser extent) into music, and I have attempted to parlay certain attitudes to my approach. I've also started looking more thoroughly at other mediums (paintings, sculptures, etc), pieces that I have long since admired but never really assessed, hoping that any insight attained would translate to better photography. More importantly, I've tried to view works that did not necessarily conform with my taste, but could nevertheless prove educational.
Some factors remain elusive, at least for little ol' brain addled me. For example, as I mentioned recently in another thread, I love a lot of the New Topographics material. When I first picked up a Stephen Shore book, I had not heard of him, and I certainly did not have any allegiance to his work. I could have easily put the book down and muttered, "Glorified snapshots." But instead, I found much of his work captivating. And earlier this year, I learned of Lewis Baltz after coming across an exhibition of his at one of DC's numerous museums. Again, some would claim 'snapshots,' but to me there is much more; and perhaps there are underlying objective elements of 'art' that elevate these photos to a higher level. If they exist, they are not overt, and this is why I consider this some of the most challenging photography.
Look, I realize we're a cynical lot, particularly in a culture where stupidity is increasingly profitable, if not deified (see RossJ, I can be arrogant too), so we are cautious not to be duped by what clothing the emperor is wearing. But Gursky (getting to the main topic, finally) did not enter the art community by being the hitherto unknown lead in a leaked sex tape or by being a contestant on some unfathomably idiotic reality TV show. And whether Gursky's "Rheine" photo appeals to one or not, Allen's advise to at least study, and not automatically toss aside, why it is valued artistically, not just monetarily, deserves consideration (There is also some socioeconomic questions that arise too, but I will restrain from any socialist-inspired ranting).
Personally, the Rheine photo did not hit me; I've seen other things by Gursky that I liked much better. This said, it has 'grown' on me, and why not? How many times have we heard a song one, two, or even five times before we started to even notice it, only for it to become one of our favorites.
At best, we can identify certain ligaments, but the imposition of absolute rules, well, I actually find this anathema to art, and the attempt to delineate rules and personal creativity is arguably where much of the debate and contradictions reside, especially since measured principles and individual expression are not exclusive of one another.
As for me, I'm just going to try to go out in take photos that make's my mother proud, or that will earn me a few million.