I'm still blah on the definition of abberation. The reason that "Abberation" is so ambiguous is because it's not defined by what it is, but by what it isn't. For example, the 5E version basically says it's anything that's not humanoid, natural, or naturally-derived (as many fantastical creatures are). Presumably it also excludes undead. What does that really say about the category of creatures it includes? Nothing. Furthermore, who's to say what the "natural world" is? Maybe Beholders and Mind Flayers are part of the natural world, albeit a freaky one.
That aside, I liked the actual creature write-ups (though Aboleths bore me). Specifically, I really like the idea that the effects of a Beholder's eye rays are basically a finger-print. Also, while causing players to freak out is cool, I'm glad to see Illithid brain-eating not be considered an instant death. I find it a lot more fun to have players yelling "get it off, get it off!" over a couple of rounds (with increasing effect) instead of being held hostage to a bad roll.