Reginald Shepherd has been very helpful in his post "Defining Difficulty in Poetry." He distinguishes six forms of difficulty, from least to most difficult:
1. lexical: What does this word mean? Or what does it mean here?
2. allusive: What is being alluded to here?
3. syntactical: Can the syntax be parsed here?
4. semantic (including figurative): "I don't understand this poem." (Or "I don't understand this metaphor.)
5. formal: What kind of poem is this?
6. modal: What makes this a poem?
Shepherd concludes: "When people call a poem difficult, they are generally experiencing either semantic difficulty ('I don’t know what this poem is saying') or modal difficulty ('I don’t recognize this as a poem').
This summary is intended to get you to read the original post! (And also to outline the terms for myself, so that I can refer back to them later.)