I've run several hundred hours of 5E, both in the playtest stage (probably around 400 hours), and since the PHB was released (about 80-100 hours) in both home games and D&D Encounters. I can say without a doubt that monsters have not been this deadly since AD&D 1E. True, their numerical stats are lower, but so are the stats of the PCs, but - and here is the important thing - they are proportional (more or less) to each other, with roughly the same ratios at 1e/2e. So no, you won't have superpowered munchkinfest characters with unhittable ACs, attack bonuses that make the die roll irrelevant, and save bonuses that ensure an always successful save in 5e like you had in 3.x. Instead of that, PCs gain new abilities that allow them to do things they couldn't before.
Now, while the playtest monsters were admittedly underpowered, the ones in the MM are definitely not. I've had TPKs (and not just kills, FAST kills) using ochre jellies, black puddings, ogres, trolls, perytons, troglodytes, and pit fiends with six summoned bearded devils. The pit fiend and bearded devils took down a group of 7 level 18 PCs in 4 rounds. I know, the pit fiend ONLY has a 19 AC, and it did take some damage, but the pit fiend's 4 attacks, ability to use wall of flame, magic resistance, and fear aura were crippling to the group, and the bearded devils poisoned group members with their beards, and inflicted infernal wounds with their glaives. The really cool thing is that the bearded devils (which are CR3) were a threat to level 18 PCs when paired with the pit fiend and played intelligently by the DM. This same encounter in 3e would have been mowed through in 2-3 rounds by 7 level 18 PCs, with hardly a scratch. My players still are terrified of not only pit fiends, but bearded devils!
The one dragon encounter I ran ended with the party (6 level 12 PCs) fleeing from an adult green dragon's lair. The green dragon only took about 80 damage, but its natural abilities and lair actions put the group in incredibly compromised positions within the first two rounds, and it killed 3 characters before they fled. Yes, the PCs did use some buffs (which were almost all concentration spells) before they entered the lair. This same encounter in 3e would have ended with a dead dragon inside two rounds as casters went nova and with all the ridiculous stacking buffs that would have been used beforehand to ensure the dragon couldn't touch them. That's not heroic or epic, its just boring.
So I don't buy your assertion that 5e monsters are weenies. Monsters haven't been this nasty since the early days of D&D.
