Argyle King
Legend
Difference is, I can play a 4E Fighter (Slayer) alongside a 4E Warlord and run into no problems. Heck, I can play a Fighter (Slayer), Fighter (Weaponmaster), and Fighter (Knight) all at the same table without any issues. I could not play a 3.0 Paladin and a 3.5 Paladin at the same table without issue. 4E Essentials was not an overhaul of the system or even much of a tweak of the system, it was literally more options for the same system.
...and yet there needed to be errata to pre-Essentials feats for them to not be broken with some of the Essentials classes.
Looking at one book doesn't tell the whole story; neither does only looking at something like class. Monster math is something which changed a lot over the course of 4E. While I could use the earlier versions of monsters, the results usually weren't very good unless I did some work to update them. The same can be said of using 3.0 monsters with 3.5; they would work with the game, but I needed to do work to update them if I wanted them to work in a manner consistent with what I wanted out of the game.