By my own view, something is art if it takes a skill or ability that others cannot replicate to create.
The old masters were named so as they created stunning pieces that were not only visually stunning, but fantastically complex to create. The cistine chapel probably being the best example of thi, but Michaelangelo did not do all the work himslef, lets not forget!
Some of the works we see today, and the price people are willing to pay for it seem a little skewed. People will buy it because of the artist and not necessarily the piece, and this is what sends the artworld into a different place to reality.
If you or I could re-create the piece, what then makes it art? (tracy emine pieces spring to mind)