When previewing a document within the Compass UI, array indexes and object keys are rendered the same and the only visual feedback differentiating the two container types is in the top-most line ("Object" vs. "Array (#)").

I realize BSON arrays and objects have identical structures apart from their BSON type (in that array indexes are numeric strings), but using slightly different formatting for array indexes might help users in situations where the types are easily confused (e.g. objects with numeric field names).