The Ghent Workgroup (GWG) has published an interesting poster:
On the front page the GWG is presented. On the back nine reasons to preflight are listed (compiled by belgian VIGC). Each topic is illustrated with an eample.
This page can be used to check how certain PDF definitions are displayed in different viewers (Acrobat/Reader, Apple Preview, etc.) and output devices (printer, proofer, imagesetter).
In addition one should check this PDF with a preflight tool in order to also detect the problems which are not visible. For this the preflight profiles GWG2012 or GWG2015 (part of Acrobat DC Pro, Callas pdfToolbox, Enfocus PitStop Pro) and of course the preflight profiles for Acrobat Pro from PDFX-ready are highly recommended.