The distinction between the standard representation of '{}' as an array with zero dimensions and nonstandard representations as a 1-dimensional array with zero elements has come up in a couple of contexts on the IRC channel recently.
Just to add to the listing of annoyances.
The following makes me want some way, in SQL, to create an empty 1-dimensional array ...
SELECT array_agg(e_arr)
FROM ( VALUES (ARRAY['1']::text[]), (ARRAY[]::text[]) ) v (e_arr);
--ERROR: cannot accumulate arrays of different dimensionality
We moved the goals posts nicely with bac27394a1c but not being able to mix empty and non-empty arrays is problematic. Ideally an empty array could become an array of any dimension on-the-fly so that if even explicitly dimensioned input to array_agg is 1-dimensioned then all empty arrays would be promoted to 1-dimension and the resultant output would become two dimensional. unnest'ing such a structure would pose its own challenges though...
