The to-hit bonuses are too low, and hp can be way too low. If you've run the blind ogre fight in Blingdenstone you know what I mean. He goes down way too fast, and there is never any palpable sense of danger despite fighting what should be a deadly battle against a physically dominating foe.
In my experience so far with 5E, fights against a lot of simple enemies (even far more than the encounter guidelines suggest) work out very well. The streamlined rules and monster stats make gameplay fast, while the sheer enemy numbers and their possible lethality (all they need is a few lucky rolls . . . ) make it dynamic, interesting, and suspenseful.
Small groups of even supposedly powerful enemies(elites), on the other hand, are dominated by the PCs advantage in the action economy. The enemies are too fragile to last long in a slugfest, and too simplistic and streamlined to give the fight any depth or excitement.
In my opinion, that's a major flaw in monster building so far. But it's a very early playtest, so I don't fault them for it. I'm sure they'll learn a few things from the playtest and give us more refined enemies in later iterations.