Perhaps WotC hasn't been clear on it.
I think Wizards have been moderately clear on it, but people are distorting the facts to make their own points. What has confused the issue is the future plans of Wizards, which have been changing recently.
Here's the easy one:
What is D&D Essentials?
The answer is this: ten products that stores selling D&D should always have in stock, which provide all (most) of the material needed to play a D&D game. The ten products are
* D&D Starter Set (Red Box)
* Heroes of the Fallen Lands
* Heroes of the Forgotten Kingdoms
* Dungeon Master's Kit
* D&D Rules Compendium
* Monster Vault
* D&D Dice
* D&D Dungeon Tiles Master Set: The Dungeon
* D&D Dungeon Tiles Master Set: The Wilderness
* D&D Dungeon Tiles Master Set: The City
So, that's the D&D Essentials line
in its totality.
However, that's not what most people are debating. What they're debating is the
D&D Essentials Design Precepts. You see, while there won't be any more D&D Essentials-branded products, there
will be products that contain material that corresponds to the designs used in Essentials. The doomsayers believe that there will never be any support of the older PHB1-3 material, it'll only be for the "Essential"-type classes.
What the heck are the D&D Essentials Design Precepts? Well, it's a heading I've just invented to describe the difference between pre-Essentials 4E design and post-Essentials 4E design. And basically it boils down to this:
It is okay to design classes that don't all correspond to the strict feat/encounter power/at-will power/daily power progression first seen in the Player's Handbook.
In actual fact, you can find the first example of this in PH3 with the psionic classes which replace encounter powers with the augmentable at-will powers. Essentials just takes it further: it's okay to have a fighter class that doesn't have daily powers, instead it gains class features. It's okay to have a fighter class that doesn't have lots of encounter powers, instead it just gains additional uses of its sole encounter power.
This is actually pretty liberating design, but it doesn't come without its pitfalls. Most obviously, if you play a Slayer (one of the Essentials versions of the fighter) and pick up Martial Power, although you can use many of the feats and utility powers, the encounter and daily powers in that book are useless for you. It also has implications with multiclassing for a similar reason: the classes aren't all built on the same structure any more. (Well, this was true of PH3 psionic classes too).
The big question going forward is this:
Will Wizards only support the Essentials Classes in the future? That's the big unknown. I take it as given that future classes will be more free in how they approach their feat/power structure (and thus might be termed as "essentialized", but I consider it quite unlikely that Wizards will *only* support those classes in D&D Essentials. They've got a lot of players who don't restrict themselves to Essentials. It's a pretty good way into the game, but it isn't the entirety of the game.
Cheers!