For reference, an S-expression based syntax that could be used for this example:

    (todo "housework"
      (item :priority high "Clean the house.")
      (item :priority medium "Wash the dishes.")
      (item :priority medium "Buy more soap"))

Your syntax certainly has advantages, but one can easily prefer S-expressions in this case without appealing to vague values like "aesthetic purity," nor are there any blatantly obvious "practical" downsides to using a more fully parenthesized syntax.