New Essential based content provides:
Feats. Unless the feats are reprints, they are generally usable by any character, old or new. (So far anyway. Some of the stuff from Dragon isn't necessarily usable by everyone as it is more specific to Essentials in some ways). Since the classes use the same class but new builds, unless the feat refers to specific class features, any class prereq feats would be backward compatible.
Utility Powers. All the classes get their choice of utility from any book, so at the very least, any new utility powers are support for all versions of the class.
Daily Powers. The marital classes don't get dailies, but with Martial Powers 1 and 2 in addition to Dragon content from day 1, the martial classes have recieved a ton of support. Everyone else gets dailies.
Encounter Powers. This one is a bit iffier. Most of the classes get no new encounter powers. The wizard does (which means the Heroes of Shadows with it's new schools for the mage wizard will be packed with stuff usable by all wizards) and while the warpriest cleric doesn't get to choose which encounter powers it gets, those encounter powers are available to the older versions of the clerics.
So, for the most part, the lack of new support is:
(a) For classes not featured [which would be true with or without Essentials ... although it's unlikely there will be an Essenial Seeker/Runepriest as they would have been new builds of ranger/cleric had they come out later, and it's unlikely for a class to be featured outsid of ragon if it doesn't get some sort of Essential treatment]
(b) Very focused help for existing builds (like the beastmaster ranger), especially outside of Dragon. Then again, how much did Martial Power 2 give to beastmaster rangers? It was mostly focused on the new builds, with a few bones thrown to the old builds.
In general, a new Divine Power book, for example, would have new builds for the existing classes, feats and powers that mostly focus on the new builds with a bit of help for older builds. By comparison, the paladin/cleric from essentials comes with some powers and feats that can help the old builds, but mostly focuses on new builds and powers that support those new builds. Now, it did leave the Avenger, Runepriest and Invoker out in the snow, but that was because instead of focusing on a power source, the books were focused, much like the PHBs, on an assortment of classes. In terms of help for the classes featured, it was perhaps a bit less than what you'd get out of an X Power book, but then again, most of the "Race X + Class Y" powers [some of which were even specific to "Build Z"] were going to be useless to most players, at least until they had a chance to build a new character.