As an avid Art History buff, I very much disagree with a lot of this. Just as you probably have some fairly nasty things to say, I'll avoid any serious flames, but I will make one point:
I do agree that art is extremely hard to define, but I do believe that reasonable people can reasonably agree. In fact, everything follows this: you can't prove I exist, as it's theoretically possible that a computer sitting in a very, very windy room happens to be turning on and having the buttons pressed by gusts of wind in the precise order necessary to open up and type this post. That's possible... but reasonable people can reasonably agree that's unlikely. Similarly, when almost all serious art critics agree that something is one of the most important works of art of all time, or that something is utter garbage, that general consensus of reasonable and educated people generally means something. Not always, but usually.
But this is a minor difference: I do think that art has some concrete form, but I do agree that it isn't absolute. I certainly wouldn't subscribe to your particular form of Dadaism, but I also wouldn't insist that art could be defined by a single blurb in a dictionary.
No one has to agree with me on this one. It's all up to the individual. In general, do "art critics" (you'll have to excuse my distate for that phrase) agree on the importance of one artist or another? Absolutely. Are they wrong? No. It's easy to point out whether an artist, over the course of time, heavily influenced others who followed. But that doesn't mean that it's "quality art". My point is that there's no such thing. Everything is on a personal level. I strongly dislike Picasso but I appreciate the fact that he changed the art world for 50+ years.
And you have to remember one constant throughout art history: with few exceptions, "general art" (meaning many of the people we now adore) were considered absolute crap during their own time. The impressionists are just one example of this. They (Degas, Monet, Manet, etc. etc.) formed together because they were the laughing stock of the Paris community and then proceeded to create some of the "greatest art" of their generation. Of course, this "greatest art" was only identified as such 50 years later.
I think we can both agree that taste is completely relative.
Not completely, but largely. I think a good example would be the human form; throughout history, what has been deemed "attractive" has changed in many ways. Obesity was considered attractive, then near-anorexia. Lighter skin, darker skin, long hair, short hair, and so forth.
But a few things DO remain constant. It's been shown that in basically every culture since anthropologists began tracking such things, men with a shoulder-to-waist ratio of about 1.5 are considered the most attractive; for women, a waist-to-hip ratio of about .7 is considered attractive. Keep in mind that this is very precise -- not .8 or .6, and it's EVERY culture.
What this suggests, to me, is that there are some things about taste that change over time, and some things that are programmed into our genes. In the same way, I think much of our taste in art may fluxuate, but there are some things that have remained constant.
The easiest examples: almost all art that's considered highly significant had a complex philosphical, intellectual, emotional, or thematic intention. Another simple thing: it's interesting to note that almost EVERYTHING considered "high art" -- be it modern painting, classical painting, sculpture, opera, or great cinema -- are almost universally loathed by children and teenagers. If they hated old art but loved modern, maybe you could argue that this is a consequence of older people "not being with the times," but the uniformity of the disinterest across all genres and time periods suggests that there are some common trends across all art forms and historical periods. I'm not even going to go into what I think those trends ARE, because it's obviously extremely complex, I'm just saying they apparently exist.