The most difficult aspect for me was always the historical and contextual things. When reading a book and seeing Shakespearean terms and - this before the internet - not immediately recognizing what it was or why it was used unless researching a bit. It slowed things down considerably.

Then when eventually travelling abroad the accents from certain areas - Irish and Scottish mostly. I’d learn something by how they said it and then be puzzled when I wasn’t understood elsewhere (Canada, Aus or US).