Great, rollicking thread.
First off, I'd like to thank David for connecting the disparate dots and coming up with the Selective Latent Image Manipulation Technique (SLIMT) method of reducing negative film contrast. You should get a Hero Badge for that one!
As for the discussion at hand, everybody is right, because art cannot be codified or defined by universally acceptable rules. There is no one path.
Picasso said, "Good artists borrow. Great artists steal". Would he have invented cubism without experiencing African ceremonial masks and statuary? Then there's Warhol, who took a simple idea and milked it for all it was worth while the new business of investing in art instead of the stock market was beginning to flourish. Lots of wiggle room between those two.
As long as people define great art by its investment potential, it will foster a system which races to the new, the shocking, or the avant garde. As the above examples show, you can do this based on a firm foundation of skill and deep insight, or have a single good idea that the art investment market adores.
There will always be people searching for the new, ready to push it into the fray with the best marketing they can muster. Unfortunately, only great lengths of time (as in hundreds of years) will answer the question as to wether it's great art or not. In the meantime, we debate!