Hint: Look for a famous song. (Additional note: This is a slightly reworked 2nd version of this puzzle. Edited to remove some potentially questionable content. If you've already solved this meta once, the meta answer is still the same. Thanks!)
The meta answer is OLD MACDONALD HAD A FARM. (Hint: Look for a famous song.) The puzzle's title was "Stock Characters." The word "characters" can refer to letters, and "stock" can refer to livestock or animals.
The four longest Across entries all had one thing in common: the only vowels they each add were E I E I O, and in that order. No extras, and no A, U, or Y.
That is, each feature the same five "characters" (in this case, vowel letters) that are the refrain of a song about farm live"stock." ... which is OLD MACDONALD HAD A FARM, the answer that satisfies the puzzle's hint.