I'm concluding this series with a brief look at the monster design so far. I'm not too concerned with the math; there's plenty of time for that to be nailed down later, and besides that I haven't actually playtested this packet yet, and since the most important thing about the math is that it works out in play, I don't consider myself terribly qualified to comment on that. Besides which, the lethality of the system will ultimately come down to personal preference, and my "too much damage!" might be someone else's "just right!"
The most important elements of monster design are 1) ease of use, 2) speed of adjudication, and 3) mechanics that emulate the fiction (i.e. how a monster "feels"). Some of these things seem mutually exclusive, but that's not necessarily the case. You can have interesting monsters that are also quick to run, if they're well-designed.
The D&D Next monsters are, by and large, too simplified. While they have some minor trait differences and have different numbers, once the blades are out and swinging a kobold doesn't fight that differently from a goblin, which doesn't fight that differently from a hobgoblin, etc. It's mostly a matter of "walk up to target and make melee attack," or "stay away from target and make ranged attack." Kobolds and goblins don't feel any more mobile than hobgoblins, which don't really have a schtick aside from "I don't get scared."
Granted, some enemies have decently evocative stuff (Kobold Dragonshield's Shield Block), but even that's yet another application of Advantage/Disadvantage. I really liked Advantage/Disadvantage when it was first introduced, but I feel that it's a bit overused, I don't like how easy it is to cancel, and shoehorning too many abilities into the mechanic reduces their scope and variety.
The stat block could definitely use some cleaning up, which would help both ease of use and speed of adjudication. Some attacks are too "wall of text" and more information than necessary is given (how often do I need alignment or languages in a fight, and consider how trivially easy it is to make that stuff up on the fly). A lot of monsters also have "Multi-attack," which basically boils down to "make these two or three different attacks," which are then all given individual entries. Why not just list a single attack (ex. Claw/Claw/Bite), with the same attack/damage values, and just say "make 3 attacks?" If you really feel the need you could specify piercing/slashing/bludgeoning damage, but that's also super easy to adjudicate on the fly. First of all, from the PC's end it usually doesn't matter (those damage types are more important for describing what the PCs themselves dish out, because things like skeletons might react differently to them). And even if you didn't list them, and it comes up in play, well isn't Next supposed to favor Rulings over Rules? Most DMs would find it to be common sense that claws deal slashing damage, and the bite is piercing. If a player asks, the DM should be able to spout that out without hesitation, or looking it up. So why clutter up the stat block with that mess?
Once a monster has more "things" describing it than Basic, you really do need to start thinking about 4E-style stat blocks. Have the things that need referencing the most jump out to the eye, and well organized (list AC prominently, preferably next to HP and any other defensive notes, then list attacks as concisely as possible). And don't just rattle off a list of 10 spells that an enemy spellcaster has. Pick a handful and write them out like any other attack: (Ranged) Ray of Frost +X to hit - X cold damage. How likely is this guy to survive the combat? He doesn't need more spells listed out than he'll use, and the few spells he does use shouldn't need to be referenced by flipping through the Spells chapter of the book. If a DM wants to put the effort into adding more spells, more power to them. Maybe at the bottom of the stat block you can suggest other spells. Most likely, they'll only be needed if this is a major villain, and otherwise they can be ignored.
I'll be honest, I haven't examined every single monster in the Bestiary. Not even close. I've skimmed through it, but I'm sure I've missed a LOT. Still, I think comments on a sub-sample will provide insight into the kind of design I would prefer be carried over to other monsters.
Giant Centipede: Cool, this guy gets poison which is as you'd expect. But the effects tread on the "4 pages (of VERY small font!) of very specific and redundant status effects that 3.x had" territory. A 5 ft reduction in speed is not going to make much of a difference on most combat rounds. It's a mild annoyance when it does come into play, but because it's so trivial it's easily forgotten. And it just increases bookkeeping. Same with a mere -1 penalty to AC. If an effect is important enough to be mentioned, its mechanical effect should be noticeable. Cut speed in half. Call it "Slowed" and add it to the list of general conditions. DO NOT create conditions that effectively boil down to "kinda slowed," or "badly slowed." That just increases bookkeeping, and the need to reference stuff in-game. Which slows down play. Similarly, don't spell out a monster ability that's "totally not the slowed condition" but that is, essentially, kinda/badly slowed. If a monster's fluff slows affected creatures down, use the darn Slowed condition to represent that!
Cockatrice: I almost threw a fit when I was listening to the Mines of Madness podcast and Barbie the Barbarian was affected by the dreaded Save or Die (turning to stone is effectively death) by the cockatrices. One bad roll should not kill your character. Fortunately, my fears were somewhat alleviated when I looked the critter up in the Bestiary and realized that they must have adjudicated that wrong. The first failed save restrains you, which allows for another save that then turns you to stone if you fail. Personally I'd prefer a 3rd save for good measure, or a general action that an ally could use to grant a free save (which wouldn't count toward the second failure), but the very presence of progressive saves is good enough for now.
Dragons: Classic example of the multi-attack problem I outlined above, but the real reason I'm highlighting this iconic monster (or group of monsters, really) is their breath weapons. They have a Recharge mechanic, straight out of 4E! I like the tension of never knowing when that breath weapon was getting busted out; could be 2 rounds in a row, or the dragon could go several rounds without getting to use it. Great way to create tension and call attention to the dragon's signature ability with an extremely simple mechanic (plus rolling dice is fun!). Frightful Presence suffers from the wall of text syndrome, though, and could stand to be simplified. Especially since Frightened is already a general condition.