Since I don't know the difference (although i do own the Essential Monster Book, whatever it is called) can you articulate the differences?
Formally, there is no difference, other than the inclusion of additional material.
Informally, there was a noticeable shift in the design for new material. It wasn't absolute, there were still some things supporting classes more generically. But, as a general rule, the class design shifted in the following ways:
- Most (though not all) PHB classes got 1-3 new "Essentials" subclasses. These subclasses usually had very few choices, and most of the choice involved in playing them came right at the start, when you chose the particular flavor you wanted to play. (Sound familiar? It should. Essentials was Mearls' influence on 4e.) For example, Cavaliers (the Essentials Paladin) could choose the Virtue of Sacrifice or Virtue of Valor, which would determine one of their at-will powers, several fixed powers later on, etc. In other words, basically what Subclass does in 5e. This was different from standard 4e, because "subclass" in 4e was mostly a personal choice, where you had the freedom to ignore your subclass benefits if you wished to and take other powers instead.
- As a general rule, beyond just the "streamlining" of build choices down to one or two choices made very early on, most Essentials classes were really simple. Some were EXTREMELY so, like the Slayer (a damage-dealing Fighter subclass), but even the Wizard subclasses were a noticeable step down in number of moving parts. If you liked what Essentials did, you'd probably refer to it as "elegant" etc.; if you disliked it, you'd probably refer to it much less positively.
- Certain features and powers were made intentionally so they could not interact with pre-Essentials content. The rule was simple: if it's a power that has a level requirement, it can be taken by anyone of that class; if it does not have a level requirement, it can only be taken by the Essentials version. (This led to one of the stupidest rules changes in the game IMO: they originally published the power Call Celestial Steed, which opened up a line of AWESOME holy mounts for Paladins, as a Utility power with a level, so non-Cavaliers could take it if they wished. Later, they secretly removed the level without noting this as errata. I was very annoyed.)
- As a general rule...Essentials classes aren't as strong as pre-Essentials classes. There are some exceptions, but overall they tend to be weaker and harder to improve than "original" 4e classes. The Vampire, for example. Awesome idea, super flavorful, very interesting concept...extremely weak class. You basically had to delve into some very specific Sorcerer multiclass stuff in order to eke out reasonable damage. The Defender options were mostly pretty good, albeit much less versatile because Defender Aura only affected things adjacent to you.
- The one* area that Essentials was, IMO, unequivocally superior to what came before: Expertise feats. Essentials gave us a series of flavorful, class-specific Expertise feats that did something interesting beyond just a dull +1-per-tier bonus to your various things. These feats were simply better than the original options, other than maybe Versatile Expertise, so I give props where they're due.
Overall, Essentials
really is the same game, as in, nothing about how the rules work or what you were allowed to do had changed
in the rules themselves. Instead, any perceived gap between "original" 4e and Essentials comes from people enforcing some kind of artificial barrier between the two, disallowing options from before Essentials came out. Which...is completely silly and not at all supported by the actual text nor the things WotC made.
*Technically there is a second thing I think was unequivocally good, but it wasn't a rules change. That second unequivocally good thing was the Elementalist Sorcerer subclass, because it, for the first time in D&D, was ACTUALLY A SIMPLE, STRAIGHTFORWARD SPELLCASTER. You were elementally magical, and you were a straightforward, no-nonsense blaster. You also prioritized Charisma and Constitution, meaning your character would be beefy, relatively well-defended, and decent at socializing--a
perfect choice for a novice player wanting something flashy and cool but also wanting to have a shot at dealing with socialization stuff.