Assuming that the writing's decent and everything is properly done...
* Good art. For a monster book, this is of eminent importance, as this is the image of the rampaging beast I get in my brain...if they look goofy or cartoony, it's less likely that I'd use them.
* A theme. Too many 'generic monster books' out there. If you do a monster book, they should revolve around one aspect of society/life/environment and the critters found there. That way, if I was taking my campaign into the desert, for instance, I could have a whole bestiary to choose form.
* A place in the ecology/society. Monsters fill a niche....which niche do these fill?
* A reason to use them. Nothing frustrates me more than paging through, say the MM2, and going "Wow! Magical Evil Cleric Worms! I like them....now, what kind of story would revove around these guys...I see WotC gives me none....which means I just need to haphazardly insert it into my own campaign whatever which way...."....there should be a reason for the party to encounter the creature above and beyond "it guards treasure."