Odd. When I heard about it I thought it was just going to be a recompliation/errataed alternative to the original core books, and when I saw it on the bookshelf, that's what it looked like. Yet I've ran across thread where people are going wiigii about how different it is from core.
Granted, we are roleplayers, so we have a tendency to get a little melodramatic.
To an extent it is. The classes are rehashes of existing themes, but done somewhat differently. The martial classes all have at-will stances replacing at-will powers. You go into a stance and use an MBA or RBA and you get basically the equivalent of an at-will. They lack choosable encounter and daily powers entirely, replacing them with a fixed encounter power (for the Knight and Slayer it is Power Strike, which adds 1W to whatever attack you just did, the other classes are similar). At higher levels you get more uses of Power Strike etc. Instead of daily powers there are built-in class features that kick in at specific levels.
Basically you need to make a LOT less build choices at start and while leveling. They are sort of "pre-baked" characters. You still get to pick utility powers as normal and feats as normal though. Overall they are somewhat less flexible. They also have less resource management due to no daily powers.
The cleric and druid Essentials builds are still more set in their ways than PHB1 classes with a lot of the options baked in, but with more choices than martial builds. The Essentials Wizard, the Mage, is basically identical to the PHB1 Wizard with a few minor tweaks. They can pick a school instead of implement mastery, get Magic Missile for free, and their spell book can hold encounter powers. Wizard encounter powers almost all get half damage on a miss now too.
That about summarizes it. They don't mention rituals anywhere in Essentials. Clerics just get Raise Dead as a class feature at 8th level. You could still take Ritual Caster, but it isn't presented in those books. Feats in general are broken up by function now instead of tier, have less prereqs, and are in some cases more effective, but generally not much different overall.
All of the material is compatible, the rules haven't changed, etc. The new classes are well balanced with the old ones, so basically Essentials is a couple splat books, or a complete stand alone limited version of 4e depending on how you look at it.
The real controversy was over whether or not Dragon and other new materials would mesh with the existing classes well or only extend the Essentials classes. There is a feeling that WotC (Mike Mearls in particular) has a hankering to just ignore the old 4e stuff and kind of take the game in the direction of being a bit more like AD&D. This may be true to an extent, but frankly I think people make too much of it. Essentials is the new shiny and has gotten more support with new stuff lately, but there are still articles covering the old stuff as well, and it is obviously not going away.
I guess we'll see how things go as time passes, but it all seems mostly a tempest in a teacup to me.