1. In general, the art is even better than the PHB. There is an almost biologic sensibility, like what might be recorded in the notebook of a 19th century naturalist. The influence from real animals on the illustrations is more pronounced than in past versions. Not in terms of direct realism, but just in that you can recognize features, poses, and behaviors that make the creatures seem more anatomically resonant.
2. Other than the green dragon, which I like, the dragon shoulders are pronounced in a way that looks somewhat ridiculous (this is worst in the red and silver dragon illustrations). I don’t care how much muscle a “real” dragon would need, it does not look good.
3. The small landscapes scattered throughout might be one of my favorite features, especially the one right prior to the demon entries. They evoke exploration and weird expanses.
4. The best and most creative art seems to be for the monstrosities. See the aboleth below, for example. The worst is for the humanoids. I like the gnoll and ogre. The bullywug is okay but then it’s also really hard to mess up a frog person. Which leads me to…
5. Most unimpressive new visual identity: goblin. Successful goblins I have seen are cute (Gremlins and Pathfinder), Tolkienesque (Alan Lee and Angus McBride), or faerie tale (Arthur Rackham and Ian Miller). This one is just boring.
6. Most impressive new visual identity: aboleth, probably. It no longer looks like a big fish with funny eyes and tentacles, but something truly alien and new. I don’t think I’ve seen anything that looks like it before. (And if you have the book, check out the smaller picture on the following page of an aboleth seemingly pulling itself over land with its tentacles.) There were a lot of other good candidates here though. See also the mind flayer (the Darth Vader style makes it really feel like an extradimensional invader) and the manticore (that mouth!).
7. Most unsuccessful illustration: tarrasque. Makes it seem about the size of a goat. Seriously, look at that thing and tell me it’s bigger than a large dog.
8. Most successful illustration: I don’t know that I can pick just one. Green dragon, pit fiend, succubus & incubus, lich, medusa, wraith, pseudodragon, mind flayer, shadow, drow, darkmantle.
9. I like this succubus and incubus picture and appreciate that the incubus was made attractive too. Likewise with the angels (particularly the deva) and the yuan-ti (which also remind me of the snake men Masters of the Universe toys).
10. The demons and undead are effective except the marilith, which just looks awkward. Zombie is kind of weak, but lich, mummy, and wraith are awesome. Mummy approaches legit scary (something about the unnatural but yet still natural head shape) and that’s pretty rare in RPG illustrations.