I have two ideas and two guesses to add to the other contributors' answers:

• German's generic compounding (especially of nouns) which allows juxtaposition of components often without any markers ( example: Erschwernis, Zulage -> Erschwerniszulage (complication, extra pay -> extra pay due to complication ) puts an extra onus on the listener as the phonetic stress patterns of the words involved are altered.

• German is a head final language when it comes to subordinate clauses. this might cause a problem because preceding lexical material needs to be retained until the head is encountered. While this does not seem to impose a significant burden onto a native speaker, a foreign language appears to be processed differently turning sentences like in the example below to a somewhat hard problem (note that there are no morphological clues and the semantics can be disambiguated by the verb only)

• German's rich morphology (at least in comparison to English) may hinder the phonetic recognition of spoken language (this is however just a wild guess)

• German's liberal word order may impede a non-native speaker to rely on patterns in sentence structure (again I'm just guessing).

example:

Die Löwin, die die Trappe fraß. (  the lioness that devoured the bustard  )

vs.

Die Löwin, die die Trappe überflog. (  the lioness that the bustard flew over  )

I have two ideas and two guesses to add to the other contributors’ answers:

• German’s generic compounding (especially of nouns) which allows juxtaposition of components often without any markers ( example: Erschwernis, Zulage → Erschwerniszulage (complication, extra pay → extra pay due to complication) puts an extra onus on the listener as the phonetic stress patterns of the words involved are altered.

• German is a head final language when it comes to subordinate clauses. this might cause a problem because preceding lexical material needs to be retained until the head is encountered. While this does not seem to impose a significant burden onto a native speaker, a foreign language appears to be processed differently turning sentences like in the example below to a somewhat hard problem (note that there are no morphological clues and the semantics can be disambiguated by the verb only)

• German’s rich morphology (at least in comparison to English) may hinder the phonetic recognition of spoken language (this is however just a wild guess)

• German’s liberal word order may impede a non-native speaker to rely on patterns in sentence structure (again I’m just guessing).

example:

Die Löwin, die die Trappe fraß. (the lioness that devoured the bustard)

vs.

Die Löwin, die die Trappe überflog. (the lioness that the bustard flew over)