Bad Art I think if we go down the route of aesthetics and beauty when judging art, we will find ourselves lost very quickly. When judging art I think it makes less sense to talk about what it is in terms of its form and much more sense to focus on its function. And if you're really looking for a word that stands for what art aims for in how it functions, that word is probably "wonder", "mystery", "awe" or some combination of those emotions. And that hasn't changed with the progression from classical to modern to postmodern art. It's simply that what provokes wonder and mystery, what moves or elevates or inspires us has changed. When art was about imitation, the ability of artists to recreate reality on a canvas or elsewhere was wonderful; and when in a deeply religious age art was about religious imagery that was awe-inspiring. But we progress and pure imitation becomes passé, religion loses it force, and further fields need to be opened in which wonder can play. This is not to say the classics don't still move us - they certainly do - but they move us in the context of their history and the variety of forms of art that exist today that contrast with them. They have not suddenly become bad art but they are not pushing the boundaries of wonder as they originally did.
What is bad art then in this context? Well, if we consider the function of art is to disturb us in a wonderful way then art which does not function or cannot reasonably function in that way is bad art no matter how aesthetically pleasing it might be, and art which does function effectively in that way is good art no matter how banal or unaesthetic its form is. Marcel Duchamp's "Fountain" is often given by people as an example of bad art. And usually the reason given is "It's just a latrine" or "It's ugly" or something along those lines. Here the issue of form and function and the context in which these interplay is absolutely crucial. A latrine in itself is clearly not art, but when a latrine is placed in a context where it is presented as art and in that very presentation serves to disturb, provoke and perplex its audience it can then function as art. Of course, once it's been done it no longer functions in the same way again. The bare latrine as great art is Duchamp's just as the Campbell's soup can as art is Warhol's.
So in a sense art is "what you can get away with" but what's interesting is to focus on why we can get away with it. We can get away with it when what we do functions in a particular way that disturbs or moves us or evokes wonder and mystery. When those emotions are absent, we cease to be able to get away with it, regardless of aesthetic qualities, and what is claimed as art is not accepted as such or at best is labelled "bad art".
(Incidentally, I haven't read the other thread so hopefully this isn't repetitive).