PostgreSQL row as jsonb "array", not "object"
You could extract the values from the JSONB value and aggregate them back into a JSON array

select 'some_table' as table_name, 
       id, 
       (select jsonb_agg(x.v) from jsonb_each(to_jsonb(t) - 'id') as x(k,v)) as content
from some_table t;


But you can't rely that the order of the array elements corresponds to the order of the columns in the table. 

Another alternative (not using JSON) would be to cast the whole row into a text value:

select 'some_table' as table_name,  
       id,
       t::text
from some_table t;


Casting a record to text should preserve the order of the columns as they are defined in the table.