I'll never forget a Karsh exhibition I went to a while back, where the plane of focus in a number of pictures was in the wrong place. Not, I think, artistically or deliberately so: I mean, in a portrait of Desmont Tutu, why would you want to focus on the arm of the chair instead of his face?
But part of the problem was that the pictures were too big, getting on for three by four feet/90x120cm. At a reasonable size, these shortcomings would not have been anything like as obvious. I was also underwhelmed by a too-big Ansel Adams enlargement from 6x6cm that I saw at the New York show (the show formerly known as VISCOMM) a few years back.
Since then I have increasingly believed two things:
1 Even the great masters make mistakes
2 Most people print too big.
Both have helped me be more relaxed about my own photography and the size of pictures I like. For me, the only reason in most cases to do a 12x15 inch print is if you have a 12x15 inch negative.