I find with a lot of more interesting topics, I have to come up with my own definition, then measure it against what various people say.
Good art, at least to me, is art that has emotional impact over time. I can view a painting by a master and it will affect me. I can hear one of Beethoven's symphonies and somehow I'm inside his head during that time.
These do not seem to be things you can measure in the moment, or at least I can't. It takes decades to sort out art that people like from art that continues to carry strong emotional impact. So, in my mind at least, there's a lot of room for subjectivity when talking about modern art. As things age, however, much of that subjectivity goes away, and it goes away for the exact reasons pg outlines.
There is such a thing as good taste, but in terms of modern art it's difficult if not impossible to put your finger on it.