Tony Vargas
Legend
MM1 monsters were balanced around a different expectation about combat. 4e tried to create more dynamic combats, that resolved rounds faster with cleaner more systematic rules, but took much longer in rounds to enable deeper tactical play. It didn't go over well. Some groups liked it, but most either found the monsters too 'grindy' (too slow to finish off) or not much of a threat (because they'd found some of the broken combos present with even the PH1, and were rolling over them too quickly for their low damage to add up).Whenever topics come up with regards to the deadliness of battles in 4E in its current form... the first thing mentioned is to use MM3 damage expressions, because monsters from MM1 & 2 are underpowered and don't cause enough damage.
I think there's also an expectation that monsters open up hard, that is, that early in the fight, they demonstrate that they're badass by bloodying a PC or something. By the time you dug up an sufficiently over-levelled or overpowered monster to do that using the MM1, you'd be looking at something the PCs have a lot of trouble hitting - and a TPK.
What it comes down to, as with so many objections to 4e, was that it wasn't 3e, which tended towards much faster (in rounds) and swingingier combats, in which it was critical to bring monsters down fast, or the party would go down fast. Later monsters had lower defenses & hps, and higher damage, to edge them back in that direction. 4e still generally retains it's more dynamic, tactical feel compared to 3e, the difference is just narrower, and it's a lot easier to get that OMG moment out of your players early in a combat (a strong 1/encounter or recharge 6 power helps with that), so they feel like they were facing a real threat.
Yep, there was power inflation, and I'm sure the monster upgrades have been, in part, to keep up with that, particularly Monster Vault, as there was a definite surge in baseline power with Essentials.It was only after the the release of all the additional books, adding in all manner of new class powers, magic items, special rules etc., that building PCs that worked synergistically with each other without even planning or thinking about it became much more commonplace. Much of this was simple power escalation due to expansion, as invariably there were always new powers introduced to the system that for any particular DMs game, had a much better chance of being overpowered in that game based upon how that DM ran it.