I'm writing these down so that we have a record of them. It's not really a complaint (didn't I help develop the format??), but some strange things that I noticed while reading the docs and actually generating listens to submit:

• use of track_* - musicbrainz uses recording for this same context. I can't remember if we decided that track was more user-friendly? Conversly, we use release instead of album.
• tracknumber is the only field that doesn't have an underscore after track
• tracknumber is a number but the docs don't clarify if it should be formatted as a json integer or string
• recordings could have multiple artists, but we only have an artist_name field. This is essentially an artist credit (but of course, that's confusing terminology). We could suggest an artist_names field in additional_data that lets us send a list (like we already allow artist mbids)
• spotify_id describes a URL (and gives an example of a URL). Spotify also supports IDs (5SPV5qSO1UNNwwBCzrNfum) and URIs (spotify:artist:5SPV5qSO1UNNwwBCzrNfum). They're all interchangeable, so it's not a huge problem, but perhaps terminology could be clarified.
• Some more suggested fields: disc number, duration (should it be formatted, seconds, or ms?)