Diablo IV is committed to the principle of player choice. It shapes how we approach many of our systems, from character customization, to the density and layout of our open world, to the design of our classes and builds. As you’ll see below, the Book of the Dead focuses on this idea of choice and what kind of Necromancer you want to be.
Choices are good. But they also require limitations. They require that you pick one thing, at the expense of another. If you can just choose “everything”, there was no choice being made.
That goes for respeccing, but it also goes for gear, such as not making affixes something you can just freely move around between items. I am not necessarily against the Legendary Essences, but it can be a dangerous path to walk down imo, if not designed well.
The choices also have to be balanced. Choosing between getting 100 power or getting 5 power is not much of a choice after all.
So to use the Legendary Essences as an example again; 1) Rares, 2) Legendaries and 3) Rares with a Legendary Essence, all need to be balanced against each other.
As far as weapons and other unique gear, Necromancers use Swords, Daggers, Wands, Focuses, and Shields. In addition to these, they are also the only class that can use Scythes.
NOOOO 
Class specific weapons. Sigh. Really bad imo.
And outside of the class specific weapons, a very limited selection of general weapons. Why not let all classes use all items. The affixes on the items would still dictate which ones you actually want to use.
What happened to Player Choice?
Bone skills are relatively physical in nature, and thus benefit more heavily from effects such as Critical Strike.
Different abilities benefitting differently from Critical Strike etc? Please be true! That would be great.
We went through a lot of iteration on how we want the experience of summoning the undead to feel for the player, and how much of their skill bar should be dedicated to this part of the class. To that end, we’ve come up with what we feel is the best possible version to ensure that players have as much customization as possible over not only their army itself, but also in which other skills they want to use to support their own unique playstyle.
If a player wants to have the maximum number of summoned units possible then they will only have to dedicate two buttons to their skill bar: one to raising Skeletons and the other for their Golem. The Raise Skeleton button is a one-stop shop to raise all the different Skeletons a player can summon, and the Golem button is used to command the Golem to use a unique effect that is dependent on the type of Golem that is currently active
Not a bad approach, but it is still hard not to think of another, relatively obvious, solution to fitting in more skills on the skill bar… Having more skill slots… 
Also seems potentially limiting if all Necromancer summons have to be either Skeletons or Golems, as well as never being able to have two different golems?
That said, it sure is good that you are able to have non-summon skills on your pure summoner build.
In addition to all this customization, we have also added the ability for players to sacrifice different components of their army for personal benefits. Next to each unit type in the Book of the Dead is an extra option to sacrifice the ability to summon that unit in exchange for a personal, permanent buff. These buffs are carefully designed and placed so that players can mix and match having none, some, or all the possible summoned units while still having relevant options to enhance their build.
Very nice.
The Book sounds good overall.
Although:
Reapers have slower attacks, deal damage in a AoE in front of them, and have a special wind-up attack that deals a high amount of damage
It would be interesting to hear, not specifically for Necromancer, but in general, how you have designed combat, so everyone wont just always pick the AoE option. Leading back to the Choices need to be balanced.
Aaand, that was it. As usual disappointing to see so little information about game systems
At minimum this was an obvious chance to show off the skill trees again.
