the 4e MM is weak on the descriptions, and it is fair to ding it on that. But it was extremely strong in other ways, in particular, why fighting monster X would likely be completely different from fighting somewhat similar monster Y.
Achieving these differences of creatures displayed in play is, in my view, a real breakthrough for D&D. And for me, it's worth any amount of verbiage telling me what the creature is like. I don't need flavour text telling me that the wight can take on a fearsome and gruesome appearance that makes others recoil in horror, when instead I have a Horrid Visage power in the statblock that causes a [Fear] close blast vs Will delivering psyhic damage and a push.
When I recently statted up some hobgoblin phalanxes (as Huge and Gargantuan swarms) I gave them the ability to recover 10 hp as a free action, with the trigger being that a hobgoblin minion is adjacent, and the power use also eliminating that minion as well as providing the heal.
On my crappy pen and paper battlemap, the phalanx was just a big boardgame counter, and the minion a small plastic token. But when I used that power, it reinforced more than any description could that the PCs were fighting a unit of hobgoblins, whose members were dying with every hit taken, and which would reinforce itself in desperation by impressing any hobgoblin soldier who happened to come adjacent.
I want my story elements to express themselves mechanically. D&D has always had this to at least some degree (werewolves can't be hit except by silver weapons, AD&D dragons cause fear when they fly overhead, etc). But 4e took it to a whole new level.
In my experience, battles against different critters feel very different in 5e so far. It doesn't require much. Ogres deal lots of damage, kobolds die in droves, bugbears favor surprise tactics...
So it sounds like this aspect of 4e has been well preserved. Which is cool. I like that these things feel different in play.
I didn't find the monster descriptions as inspiring as I do the 4e ones, but (i) the 4e ones are much more nicely formatted, making it easier (for me at least) to read the way they are likely to play off the presentation of the stats, and (ii) I haven't actually played with/GMed the playtest monsters.
I'm glad to hear a report that they play differently.