I pretty much agree with Lanaya's take. It was also an attempt at a softer reboot than 3.0 to 3.5. The goal was to make a new introductory point that never explicitly invalidated the previous content. Sure, there was a lot of errata as part of it, but there was always a lot of errata. The trouble was the writing never really knew how to play well with what came before. As a result, you had multiple versions of classes loosely repackaged as subclasses of each other that often retroactively needed to be renamed as such.

The biggest example, for me, of this difficulty reconciling post-Essentials and pre-Essentials is the Bladesinger, which came out in the Neverwinter Campaign Setting guide. Now it's not the greatest class, but it manages to emulate the sword and spell gish feeling pretty well. Now, the Swordmage came out several years earlier in the Forgotten Realms Player's Guide. It's not the same, by any means, but it's certainly as close as any Essentials class to it's o-version. But the Bladesinger isn't a Swordmage, it's a Wizard. I understand it has been a Wizard build in just about every other version of the game, but those versions don't have Swordmage as a class to share feats and powers with.