There are three main types of defects, from a modern point of view, found in Aristotle's Poetics.

First, the text itself is fragmentary. The final book, which addresses comedy, is no longer extant.

Second, Aristotle treats tragedy and epic but not lyric poetry or other literary genres of his period.

Finally, and perhaps most importantly, Aristotle addresses only a limited number of genres practiced in ancient Greece. He ignores all works in languages other than his own. Moreover, there are many types of literature which have evolved since the time of Aristotle, most notably prose fiction, which Aristotle does not discuss.