Complex text is a key component of the Common Core. Why? To make sure that students entering college are "ready to read college-level texts independently."
This post dicusses how the Common Core measures complex texts with three aspects: Qualitative, Quantitative, and Readers and Tasks, providing information on each aspect.
It also notes that the Commone Core is still a work in progress. As we move to the adoption of the Common Core standards this post helps to provide a better understanding of complex text.