I received my 5E Monster Manual yesterday and spent the evening and part of this morning devouring it. (Mmm, wood pulp! :d) It’s a seriously impressive book, giving almost every monster a page which includes lovingly-rendered art, several flavorful bits of monster lore which the DM can use or ignore freely, and a stat block. This book, like the Players Handbook before it, has just that touch of whimsy (from the “delicious squishy brains” disclaimer buried on the facia page to the outhouse mimic sketch in the index) that both 3E and 4E lacked and I have missed. (Go back and look at the original AD&D Dungeon Masters Guide and you’ll notice that at least half the illustrations were single panel cartoons nicked from Dragon magazine!)
It’s not without its quirks, of course. Many of the creatures I find the most interesting have been shunted off into “Appendix A: Miscellaneous Creatures,” by which they basically mean “beasts.” But since the category includes such staples as blink dogs, giant spiders, worgs, and all of the swarms, you’d think they’d merit a little more respect.
Also, much has been made of the lack of an index by CR. Personally, I find this a non-issue, since the DMG is probably going to have all kinds of encounter tables and the like, but WotC has since published said index on their website, and Blog of Holding has done one that’s probably more useful if you’d like such a thing.
But on the topic of CR, wow did CRs trend down in 5E! Creatures that have traditionally been unholy terrors at the “heroic” tier  such as manticores or wights, tend to top out around CR 3.  CR 5 is home of the “big league” monsters such as trolls or gorgons, and then the eldritch nasties such as mind flayers or hags start appearing in the CR 7-8 range. This is clearly a deliberate design decision, which I have a few theories about.
First of all, the encounter budget models that WotC have released so far all indicate that the number of monsters shoot the difficulty up quickly, which means that while a single CR 2 ogre would be a “hard” encounter for a 2nd level party, a pair of them would be considered a “ludicrous” encounter.  Since many DMs love to throw groups of monsters at the party, keeping individual monster CR down keeps the difficulty from going through the roof too fast.
Second, D&D has always had a certain “When do we get to the good stuff?” problem. The game’s iconic monsters, things like adult dragons and beholders and mind flayers, don’t tend to appear until 5th level or higher, while many campaigns struggle to get past 3rd due to player attrition, DM burnout, or whatever. Skewing the CRs down makes it more likely that the average group will advance to a level where the bigger, badder, “cooler” things can start showing up, hopefully sustaining interest in the game and opening the campaign to more varied scenarios than another March of the Goblins. 
Finally, bounded accuracy rears its head again: low level baddies can still hurt higher level PCs. One on one, a lower level critter will certainly run out of hit points long before a higher level PC will, but when you get a room full of them, that’s another story. Lower CR monsters fill the niche that minions were intended to in 4E, without the “meta” aspects (“Why does this goblin have 33 hp, and that one only have 1? They look exactly the same…”) So a creature’s CR is not really as important a factor in encounter building as it was in previous editions, it’s just a general indicator of a creature’s toughness.
I’m looking forward to seeing how this works in play. And after this weekend, there’ll be a two week break in my gaming schedule, so I’m also looking forward to retooling my Silver Coast game with a full range of monsters, rather than just what was available in the Starter Set. Now then, where on this map could I put the Tarrasque…?
 I’ll rant about tiers some other time. When codified as they were in 4E, I find them horrible metagamey constructs; fortunately, 5E just uses them as handy labels for the DM, which is fine.
 This means that my Summoner Conversion will need a serious retooling, probably topping out the eidolon’s form at CR 4 or so.
 I’m not sure I agree with their assessment of encounter difficulty: my players have so far waltzed through multiple “hard” encounters without breaking a sweat. But then again, my players all have years of gaming experience, so it might just be a testament to their playing skill.
 Mind you, I love me some goblin invasions. But you can’t do that every time. Nor can you make every campaign about Tiamat trying to break out of her extra-dimensional prison. Tyranny of Dragons, I’m looking at you. Didn’t Red Hand of Doom kinda sew up that idea for a while?