So another thought on the doubled hit points monster problem.
This lead to fairly easy thumbnail fixes decently early on, cutting monster hp and increasing their damage. I saw suggestions of cutting hp in half, increasing damage by 50% or 100% to provide the same level of threat/challenge in fewer expected rounds of combat.
And with progressive monster design we saw that pretty much officially implemented over time in progressive revised monster math culminating in MM3 and then the Essentials Monster Vaults.
This is all exaggerated.
The base formula for hp didn't change, except later solos tend to be 4x rather than 5x standard hit points.
The base formula for NPC/creature damage
did change, from 8 + level/2 to 8 + level. This is not very noticeable at Heroic tier, but is very significant at higher levels.
The other significant change between early and later monsters was (1) to reduce the defences of elites and solos (early ones tended to have +2 over what their AC "should" be based on role and level) and (2) to not gives brutes the -2 to attack that the DMG says they should have, and that early brutes tended to have.
If you compare Heroic tier standards and elites in the MM and the MV, you won't see much difference:
Eg MM Goblin Warrior, Level 1 skirmisher, AC 17, 29 hp, damage 1d6+2 or 1d8+2; MV Goblin Cutthroat, level 1 skirmisher, AC 15, 30 hp, damage 1d6+5
MM Gnoll Marauder, level 6 brute, AC 18, 84 hp, damage 1d8+6 plus good buffs vs bloodied; MV Deathpledged Gnoll, level 5 brute, AC 18 (19 at 6th level), 70 hp (80 at 6th level), damage 2d6+9 plus small buff vs bloodied
MM Owlbear, level 8 elite brute, AC 22, 212 hp, damage 2d6+5; MV Owlbear, level 8 elite brute, AC 20, hp 212, damage 4d6+6.
someone picking up the 4e Monster Manual, whether someone entering and just getting one and going for the first one as the most core, or someone pulling theirs off the shelf to use with MM3, always had the bad math monsters there either leading to the slogging fights or requiring those in the know to adjust the monsters for use redoing their numbers from scratch every time.
This is all greatly exaggerated, as per my examples just above. I used MM monsters throughout my campaign, with the only changes being to up their damage to MM3/MV values, and to give Brutes a better attack bonus.