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.
However this meant 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.
They had errata, errata incorporated into the online tools, and later the essentials monster vault to soft be a replacement core MM, but still for people coming in the first monster book is going to loom large as the option for their one monster source go to. This was a hit that would persist throughout the edition.
If you pick up the PDF today I believe it is the original, no online errata version, similar to how the PH PDF has CAGI with automatic pulls.