a critical eye
A critical eye is an essential tool for an artist; it's perhaps as important or more so than mastery of craft, or skill with the medium. A critical eye is what tells the artist he or she can improve, and when a piece is complete. But it should never detract from the enjoyment of a piece. It can help critique a piece, finding flaws that could be fixed to make a piece better, but more importantly it should be used to enhance the experience of a piece of art. A critical eye should be used to tell you what the artist was trying to convey and accomplish with a piece, and what elements of the piece are used to further that goal. In other words, just as a critical eye will tell you what was done wrong, it will tell you what was done right. Try to disregard flaws that are tangential to the artist's message. And understand that some shortcomings may be intentional -- if the flawed aspect was unimportant to the artist, it may have consciously been neglected to better focus on what was important to the artist.