As I've hinted in my other thread, I am about functionality first amd foremost. While most of the art in the new MM is good, the functionality of the book leaves much to be desired.
There is no index by CR so I am left skimming through the book having to look at each monster to see their CR and now what I have to work with a certain levels. I find this really annoying.
2 things about this. One as many people have already pointed out, there is already a CR list online, so if you really want it, it's there. Secondly, I have found myself needing a CR less and less over the course of designing adventures for my players. CR is and has always been a guideline, but it means even less in this edition because monsters can be viable over the course of most levels. So with this edition instead of going "Ok, what monsters are exactly 10 CR so I can use them against my PCs?" I say "Huh, what monster would be really cool to fight? Ok they're challenge rating 5, so I'll just through 5 or 6 at them to make it challenging and maybe throw in this monster that happens to be CR 8. Looks good."
I am a fan of custom monsters and the book really falls short in this deparment. It does have a section on it but it's sparse. I really like building my own monsters because my group has a habit of buying MM's and memorizing their stats so I like to keep em guessing.
As others have stated, the monster manual has almost never been a place for monster creation, and it's been known for months that this one wont be either. Why you expected it to be is beyond me. Furthermore, remember that the monster manual has always been a compendium of monsters to look up when you're either running a written adventure, or want some interesting ideas for things to throw at people. Not everybody is as deeply creative as some of the DMs on these boards, so it's nice to have something there. IT's also good to note that this time around they are really serious about the DM's Guide being the ultimate adventure creator. With a CR list, traps, monster creation, magic items, etc, you won't have to flip back and forth in the monster manual and player's handbook to create anything. It'll all be right there at your disposal.
I'm finding the actual stat blocks to be a bit lacking and they are really overusing giving monsters spellcasting. I don't always want to give everything spellcasting, sometimes I want to give them some unique ability or abilities and this book just doesn't provide that.
Ok, I had to do a quick glance through the monsters to see which ones had spell casting, because I seriously didn't remember that many. Turns out there are about 45/300+ monsters that have innate spell casting. A little bit more than I thought, but still only 15%. The surprising thing to me was how few spells these monsters had. Heck, a Mind Flayer, something I've always associated with spell casting, only has four spells total! Many have even less, and only a very few have more than six. The vast majority of creatures have some different little ability that isn't spell casting, so now you're just spewing hyperbole.