I honestly haven't had a chance to fully read through and digest this material yet, so my answers might change in the future. Just based on what I have been able to assimilate so far, these are my answers to the opening poster's questions.
What do I like?
For starters, I like that they've brought back a lot of older and more obscure creatures. Some of these I would have preferred to see stay gone - Stench Kows immediately leap to mind in that department - but the return of oldies like the Froghemoth, the Flail Snail, the Catoblepas and the Vegepygmy is nice. Heck, they even managed to make the traditional goofball Nilbog into something that's actually pretty cool.
The Hag chapter was a good addition to a species that's never really been focused on all that much in D&D. It's very nice to have the traditional quartet complete at last with the return of the Annis Hag, but I was surprised to see the Bheur Hag - in fact, I think the Bheur Hag is the only D&D-unique hag, even though Pathfinder has created a huge array of hag variants, including Blood, Storm, Winter and Mute Hags.
The new lore on Beholders, greatly different as it is to anything we've had before, is a huge step up in my opinion. It actually takes their traditional mutability and gives it more sense, whilst at the same time making Beholders feel more "alien". They're honestly been kind of a joke traditionally, in my experience; they just didn't have that same scary "oomph" as llithids or even Aboleths. The tweaks made to them are really something impressive.
A huge positive for me was the return of some of of 4th edition's unique monsters to 5e. Banderhobs and Witherlings might be the only ones to claim that position, but at least they're back at all.
Even if I do have some complaints about how they were handled, the fact they at least tried to give us some more monstrous PC racial options deserves credit.
On that same thread, at least they put some heart into Aasimar. The Volo's Guide version is a very different beast to the DMG version, and even if the "guardian angel" fluff makes me lift a slightly contemptuous eyebrow, I like the basic ideas here. The Protector, the Scourge, the Fallen, they all feel coherent and yet very different. It's taken a race I normally couldn't give much thought to and actually made me kind of like of it, so definite props for that.
What do I dislike?
Hoo boy... this is going to be ugly.
First and foremost, my problems lie with the Monstrous Adventurers section. There's quite a few things here that bug me.
As some others have said, the statement about the Monstrous Adventurers being "possibly imbalanced" feels like a slap in the face from WoTC. They've had all these months to work on this book, and they're all but saying outright they couldn't be bothered to put the effort in to make these races balanced? I managed to knock out fairly decent gnoll and goblin stats of my own, it can't be that hard! To say nothing of how "purist" DMs are going to leap all over this one sentence to justify forbidding anyone from not playing the bog-standard pseudo-Tolkein races.
I'm also less than happy that, after being led to beleive we were going to get "around a dozen" detailed racial writeups, and more "quick-use" racial writeups, implying a fairly large number of races, it actually turns out that there's a ground total of thirteen races, total. Feels like false advertising to me.
The fact one of those slots was dedicated to the Goliath is a small but nagging black spot, too. I get that there's DMs out there who'll refuse to use the content from Elemental Evil Player's Guide because it's not a printed sourcebook, and that it's no different to doing the Svirfneblin in the SCAG, but since we only have that small number of racial slots to begin with, a reprint feels kind of insulting.
The return of negative ability score modifiers. Oh, sweet, chocolate, crunchie bars, where do I begin to go into how much this ticks me off? 4th edition set up a huge milestone when it struck up the brilliant idea of supporting Round Race/Square Class by making races optimized for some classes and not penalized at others. I've seen all the arguments about how this supposedly adds greater depth, and my response to those arguments can't be posted here due to the forum's laws against profanity. Suffice it to say that I do not agree with this decision and I hope they bury it back in the junkpile of history alongside racial class/level restrictions and mechanical support for Alignment, where it belongs.
Kobolds... ai-yi-yah, my poor kobolds. I was expecting them to retain Small and Sunlight Sensitivity, because those are both fairly balanced and reasonable racial penalties under 5e mechanics. But -2 Strength on top of that? With only a single +2 to Dexterity to compensate? This was awful enough, especially given the 4e version was so beefy despite its Small size, with +2 to Dex and either Con or Cha, with +2 to all defenses against traps, and the ability to grant a free 1-square shift to all allies once per encounter. But the cherry on the top was the Grovel racial power. Mechanically, it's fairly strong, I can almost, kind of, see why it ended up being the only PC race with three negative racial traits as a response, but the flavor is awful. It just feels like a tremendous slap in the face to anyone and everyone who likes kobolds. Yes, they've always been the underdogs, but mechanically forcing them to cower, bed, whimper and snivel? That's never been on the table! Even the Pathfinder variant racial trait "Jester" is both A: easily renamed, and B: a simple mechanical boost of "+2 to Diplomacy and Perform checks, and these two skills are always class skills for you". Simple, clean, elegant, effective, and completely non-humiliating.
Next to kobolds, the Gnolls were my biggest disappointment. Gnolls have been playable in literally every. Single. Edition. Since 1e's pamphlet "The Orcs of Thar", through to AD&D's "Complete Book of Humanoids", to no fewer than four sourcebooks in 3.5 (Monster Manual, Unapproachable East, Savage Species, Races of the Wild) and then, 4th edition's "Playing Gnolls" in Dragon Magazine #367. I've had an affection for gnolls before then, but that simple article truly cemented them in my heart. It took what had always been a fairly bland "next step up the food chain of evil" monstrous humanoid, and it gave them a character unique and distinctive. It acknowledged their quo status as chaotic evil bandits and raiders, but it also wasn't afraid to explore their potential to be something more, to have roles as their own kind of points of light, or even be heroic characters. That 8 page article stands unchallenged to me in terms of quality gnoll lore. 5e gnoll lore, to me, feels like they took all the "demonic origin" related lore of 4e, excised the "not always evil" parts, and doubled down on mindless brutality and demonic corruption. Although I don't deny Volo's Guide has its uses for fleshing out packs of the Butcher's Brood, as a whole, it's inferior to what came before and I will happily stick with 4e for my lore-needs.
The Final Verdict:
As much as I ripped into Volo's Guide beforehand, I'm still going to buy it. Many of the monsters are actually fairly well done, and at least we're seeing some effort to expand the racial options. I'm bitterly disappointed at yet more evidence of throwing the baby out with the bathwater in the wake of 4e's hatedumb, but the book, as a whole, still has enough positives to me that I will happily buy it. I just won't be singing its praises unequivocally like I was expecting to.
