I've been running a 5E game for two months now. All of the core books are out, so we now have all the information. I'm getting a feel now for what the designers were going for, and why it was not to Monte Cook's liking. But I was never a fan of Monte Cook's design philosophy.
In adopting BECMI material to the new edition I'm also seeing what was originally said, way back when, about this being an edition to bridge previous editions... it's got some truth to it. I'm looking at crunch that hasn't been relevant for 20 years and realizing that with the way ability checks now work, it could be layered effortlessly on top. The DMG gives a fantastic section on converting or building monsters. The DMG is just a fantastic book all over, taking the best lessons that Gygax and Robin Laws had to teach to heart.
For the Monster Manual, I'm mixed. The hooks for each monster take the old 2E Ecologies and build on them with an actual understanding of themes and narrative importance. Some have said that the new monster writeups aren't as thorough as they were in 2E, but I invite anybody to go back and actually look at the formulaic Habitat/Society and Ecology entries during that era. It was largely filler. Where the new book stumbles, I think, is that all of the monsters are significantly the same in terms of mechanics. There isn't the variety of actions there were in 4E, and bonus actions are rarely given. These creatures will claw/claw/bite until they've had it, or, on a good day, slam.
But maybe it isn't such a bad thing to leave monsters uncomplicated. Maybe I shouldn't be in love with my monsters' statblocks and wanting to show off all of their interesting abilities. If you look at the monsters through the lenses of storytelling, then this is an amazing toolkit. And by leaving the combat action simple, combat encounters are sped up enormously. It isn't chess anymore.
I'm pleased. I thought it would be a retread of 3.x with tacked-on retro elements and a saving throw system just waiting to spiral out of control, but I was wrong. I have to hand it to Mike Mearls and Jeremy Crawford for doing a pretty good job of living up to their campaign promises. I think we all expected them to be bollocks.