An interesting discussion - thanks to all who have contributed.
I'm not an expert in this by any means, but I am trying to learn about these things.
From what I understand, a lot of the reasons why art is considered great is because of its context. For example, Raphael's "the nymph Galatea" is considered a great work of art because of the novel composition and arrangement of the figures which previously had been static and symmetrical. There is a symmetry in Raphael's work, but he still managed to produce a balanced work that also pointed to the subject of the piece. It was conventional in that the arrangement and composition was pleasing, but it was novel because the figures looked more "real" than any artist had produced before.
Likewise, Carravagio was considered a great artist because he didn't like the pursuit of perfection in his subjects - he painted famous figures in an even more realistic way (like his "Doubting Thomas" where an apostle - previously depicted as perfect was shown as a surprised old man - an ordinary human like he was).
Comparing both pieces, Raphael's work looks fake and unrealistic (particularly those Dolphins!), but these were painted years before, so Carravagios work represents a new development in art.
But the search for novelty can be a waste of time too. I'm sure there are artists who did "new" things that are now working at real jobs with their art forgotten. Art is full of trends as anything, but great art can survive them better.
I guess this means that for me, context is important to the work, but not necessary. Let's face it, if HCB had existed later and only produced his work now, he wouldn't get anywhere today (remember the Flickr discussion where someone put his "Mario's bike" up and was criticised because it wasn't sharp enough?) but at the time, his work represented a new approach to photography in the sense of social realism. Now, it would be derivative and people would say, "so what? It's all been done before." I still think that his work is good because of his composition, but from what I understand, his work was cropped an awful lot.
To me, it doesn't detract from the art. The final result in terms of its context is enough, but every artist uses their tools to their best ability to produce the work that they think is best. I'm happy to crop to produce what I think is a better picture, but whether I'm right is an opinion that everyone is entitled to have.
Of course, when a keeper comes up perfectly framed from development, it's wonderful!