J.H. Prynne on the difference between difficulty and obscurity in poetry:
It is useful to distinguish "difficult" from "obscure." When poetry is obscure this is chiefly because information necessary for comprehension is not part of the reader's knowledge. The missing information may be specific (a personal name, say, or some tacit allusion), or general (an aspect of religious belief, say); and finding out this information may dispel much of the obscurity. When poetry is difficult this is more likely because the language and structure of its presentation are unusually cross-linked or fragmented, or dense with ideas and response-patterns that challenge the reader's powers of recognition. In such cases, extra information may not give much help. Alexander Pope's The Dunciad (1728-43) is now obscure but not especially difficult; Wallace Stevens's "Thirteen Ways of Looking at a Blackbird" or "Sunday Morning" (both published in his Harmonium, 1917 [sic; see comment by Steven Fama, below]) are difficult but mostly not obscure; Eliot's The Waste Land (1922), or in long retrospect the wuti ("no title") poems of Li Shang-yin (c. 813-58), are hard for readers because they are obscure and also difficult; indeed, their difficulties are deliberate and integral to poetic method; compare also the later case of Bada Shanren (1626-1707) [...] In a later case there is pastiche obscurity as a quasi-parody of eclectic learning: compare Lie E (1857-1909), Lao can youji (1907) [...] In such combinations, each type of hardship for the reader makes the other type harder (and, it may be, more rewarding) to deal with and understand.
- from note 1, "Difficulties in the Translation of 'Difficult' Poems," Cambridge Literary Review, 1.3 (Easter 2010), pp. 151-66
Addendum: There is now a website devoted to "difficult" poetry: http://www.arduity.com/