Hold on...is Heroes of Shadow part of Essentials? I've been hearing a lot of connection between the two, but I was under the impression that Heroes of Shadow was another core product, like the PHs and Power books.
Essentials is the new core. Well, it's more core than core, it's /essential/.
One of the challenges of publishing RPGs is that the main book, with the rules you actually need, always sells more than the suplements. WotC has been trying to overcome that challenge. In 3.x, they split the rules you needed to build characters over multiple books, and made sure that most suplements had something to make some characters more powerful - a feat, an item, a rule, something - the idea being players would buy the books to beef their characters up.
DMs responded increasingly with 'core only' games.
In 4e, WotC went ahead and slapped 'core rule book' on everything that wasn't an actual setting book or module. 4e didn't sell quite as well as they'd hoped.
Aparently, one complaint is that there are too many core books coming out too fast. So, now, we get 'Essentials,' a set of only-10-books-we-promise, that give you the minimum you need to play legitmate D&D games.
If it works, Essentials will sell well, like core used to, but subsequent suplements will fall off, just as non-core suplements always have.