Haa... Back in the good old days filler meant the material anime kept adding to the source (be it manga or novel), to be able to keep running infinitely, when the anime was nearing the current latest chapters of the source material. Like the infamous Naruto or One Piece filler arcs. If you say filler is anything any individual watcher feels doesn't contribute enough to the main story, then pretty much anything can be filler and the word loses all meaning.