Diminutive Bestiary.
I think for me the biggest disappointment in the core rules (arguably the only one) is the diminutive size of the bestiary. I would have expected at least another dozen or so monsters. No zombies, ogres, elementals, golems, werewolves, etc? Admittedly, elementals are partially covered in the magic section but not in enough detail. A real shame for me. Sure, I can write them up myself but a dozen more monsters with maybe half a dozen more pictures would be about right. Also, a page for Demons and a page for Dragons is hardly sufficient for such core monsters. Two pages a-piece, covering lesser and greater variants would surely be a sensible inclusion.