The January 8, 2007, issue of The New Yorker, contains an article by Malcolm Gladwell
called "Open Secrets." As is so often the case, Gladwell is happy to have discovered a counterintuitive concept that allows him to discuss everyday issues in a surprising way (the focus of the article being Enron; Gladwell has also been adding new thoughts on the subject to his blog).
Now an article about Enron may not seem like a good place to steal ideas for thinking about literature, but here's the bit I find suggestive: "The national-security expert Gregory Treverton has famously made a distinction between puzzles and mysteries. Osama bin Laden’s whereabouts are a puzzle. We can’t find him because we don’t have enough information. ... The problem of what would happen in Iraq after the toppling of Saddam Hussein was, by contrast, a mystery. It wasn’t a question that had a simple, factual answer. Mysteries require judgments and the assessment of uncertainty, and the hard part is not that we have too little information but that we have too much."
This might be a useful way to think about "difficult" poems: some of them are puzzles, providing incomplete information, leaving the reader wondering about the unstated details. Others are mysteries, and the problem is not lack of information, or information that has not been given, but an excess of information, an overdetermination that leaves the reader wondering not what has not been said, but how to sort out what has been said.
I don't have any examples yet, but perhaps I will return to the issue someday soon, if I come across some poems for which the concepts seem to be useful tools (that, of course, is the test of any such concept: is it a useful tool?)
(I should add that I have not read the review that I provided as a link with Treverton's name, but I hope to have a chance to do so.)