I guess there were thousands of "master DMs" out there during the public playtest, then?
Seriously, I think that this is nonsense- 5e looks fine, plays fine, runs well (and fast and furious) and is not all that hard to adjudicate for anyone with a touch of judgment and common sense. At least, that's how my group found it.
This, on the other hand, is very true. You can either run 'em as is or turn them up a notch, but I wouldn't worry too much about it.
One thing you might want to do is include a "proficiency bonus" on attacks for some monsters, and maybe add a few hps here and there. Or just let it ride.
When my group fought an ogre at 1st or 2nd level, they killed it in two rounds.
This is very much a matter of playstyle. Some groups love save or die; some hate it. Either way, I don't recall a ton of these kinds of effects in 5e, and most are "fail two saves and die", at least the last time I really went through the Bestiary.
I doubt there were thousands of people playing in the play test at all. Those big numbers Mearls likes to throw around are just the number of people that downloaded the packets, that includes people like me that couldn't get anyone to play it.
There are several monsters that have instant death effects. Just thumb through the most recent bestiary. They are also not warned against using them in random encounters. Which means some DMs will make the mistake of throwing them out there and angering their players because instant death for no reason is never fun. (I played 2E for quite a while so I know what I'm talking about)...