Heroes of Shadow is going to be written Essentials-style, which means that instead of giving you the basic foundation of the class and a plethora of options to choose from in order to build upon that class in a way that pleases you, Wizards will instead give you limited control over the class's abilities and provide fixed abilities that will deliver on a concept that they envision. Hopefully, its a concept that you like.
Think of it like this: a fighter has six fighter talents to choose from. Each one gives you some benefit that helps build upon a concept, such as a gladiatorial fighter, a semi-pugilist type, a sword-and-board knight in shining armor, a guy with a big fucking weapon, and more. From there, you still get to pick numerous exploits that let you do things that you envision your character doing, on top of the simple "swing your weapon" maneuver. While a few people can't wrap their heads around daily exploits, the point is that you get a lot of flexibility.
With Essentials you don't get the fighter, you get the knight and slayer, which are basically inflexible fighter subtypes that either use a one-handed weapon and shield, or use a two-handed weapon. Sure, you get to pick a pair of stances at level one, but no matter what your encounter special lets you lump on another damage die. Want to be able to hit them in the legs and trip them? Too bad. Want to be able to smash an enemy in the face and daze them? Nuh uh. It's unfortunate that I could build the exact same concepts just using Player's Handbook. The difference is that I have more control over how the character grows.
Not all the classes function like this, however. Mages get to pick a magic school, but aren't restricted on their spells. If I pick Pyromancy, nothing says I have to pick any spells with the Fire keyword at all. This is really strange because you'd think that if they were going to pigeonhole you by school that there'd be at least some restrictions. I don't subscribe to the belief that only spellcasters need to be complicated (a level 1 mage has to pick nine spells, by the by), and I think it's great that 4th Edition allowed non-magical characters to have interesting actions and powers that they could use on a round-by-round basis.
Some people like this new direction. I'm more in the middle ground. I like a few of the classes out of the Essentials books, but namely the hexblade. Partially because it delivers at least two solid concepts, but partially because you get more say on what you can do. In other words, encounter and daily powers. Lack of daily powers, a preset encounter power that isn't even an attack, and the fact that I'm doing the same basic melee attack virtually the entire time are the reasons why I'd be bored to tears playing a knight or slayer. I like variety, and the Essentials spin on martial classes seems like more than a few steps back.
Fortunately, the necromancer and nethermancer seem to be mage schools, or at least based on the mage. This means that they'll actually get to pick stuff. The blackguard seems to be based on a paladin, like the cavalier, so she'll also get to choose more powers, as well. Would I have preferred Heroes of Shadow to be presented in the Player's Handbook format? You bet. Give me more options any day. Thankfully, when it comes to spellcasters, the Essentials format hasn't really done anything to change how they work.