The first thing that indicates this being an Essentials book is the back cover, which states:
Player’s Option: Heroes of Shadow is aimed at players who are ready to reach beyond the Dungeons & Dragons Essentials books,
Heroes of the Fallen Lands and
Heroes of Forgotten Kingdoms.
For use with these Dungeons & Dragons Essentials Products:
Heroes of the Fallen Lands,
Heroes of the Forgotten Kingdoms,
Rules Compendium
Also, the name itself was a dead giveaway, as a Traditional 4E book would have probably been called "Shadow Powers" rather than "Heroes of Shadow".
But what really makes "Heroes of Shadow" an Essentials product is in the class design and structure - the four classes presented, as well as the class options like the new Death Domain, were designed using the format seen in the other "Heroes of..." books: melee classes were given a limited number of power options, while casters had their choice from a plethora of spell-like powers.
The Essentials design structure differs from the Traditional 4E character "chassis" by limiting the actions of melee characters and reducing the number and choices of their daily attack powers. For example, by level 10, the Executioner being pure melee has no daily attack powers at all; the Blackguard as a melee/caster mix has two daily attack powers, while the Vampire and Binder have the three dailies that a Traditional 4E character would have. The Assassin (Executioner) has a daily-like power, in that they can prepare three vials of poison from a short list of four poisons they have learned by 10th level.
The other Essentials design concept is to limit choices when creating a character to make the process faster and more streamlined for
new D&D 4E players. As an example, when creating a 10th Level Vampire and selecting powers, a player is allowed to make only one choice: at Level 2, they can choose between two encounter utilities - that is all the choices on powers that a Vampire is given. Other classes have similar limitations, being able to make selections between 2-3 powers at certain Levels.
While it is true that the Dev Team has stated that Traditional 4E characters may use feats and powers from Essentials interchangeably, that fact does not transform this book into Traditional 4E content. This book has a lot of great material in it, and my grade card showed that I respected and even liked most of the content - as a reviewer, I was able to take a step back and looked at it objectively. But it was still designed under Essentials parameters and labeled on its back cover as Essentials -which makes it an Essentials source book.