Classes that can suddenly fill other roles:
Fighter, Ranger, Druid
Multi power source classes!
Multi role classes!
Different starting proficiencies (cleric vs warpriest)
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Add all the slow changes they made in preparation of essentials (like many people pointed out they made in 3.5 with sourcebooks like 'Tome of Battle'), like being able to choose between race ability bonuses.
Yes, many things are listed as options, but like many of the new feats, showing a better, more supported option (nerf to the feat that allows to change the ability used for melee basic attacks -> new classes get it as class ability; new expertise feats) is also a slow replacement.
See also the fluff support between standard infernal warlocks and infernal hexblades with their 'iconic' race: The new tiefling can get 'perfect' stats for the essential variant, but not the old 4e.
And I'm fine with it. I'm not fine with people who treat other like being dumb for comparing essentials with a revised edition. It is to similar.
If you were playing a halfing and you changed from 3 to 3.5, your weapon sizes changed. If you were a ranger, your HP changed. Etc.
If you were playing a fighter, unless you build a NEW fighter from the ground up, the fact that another version of the fighter can be a striker doesn't effect you any more than a PHB1 fighter might be envious of the battlerager from Martial Power.
All the things at the start have no impact on existing content. Those are new areas of design space that have opened up. Just like the addition of psionics which go a different direction as far as encounter powers are concerned, Essentials opens up new avenues of design space.
With 3.5 the old classes were themselves changed. With Essentials, the old classes were given new builds. Those new builds are, in some cases, radically different from existing builds. They do not, however, REPLACE the old builds. If you play 4e, with the Essential rules, outside of some powers being errata'd for wizards, and access to some new uility powers and feats ... they old classes are still the same. Ditto with races. Yes, you now have more options with the races, but the original versions are still valid, they just happen to be one way to build a character of that race, not one of many.
The reason people say addition not revision is that 3.5 took the existing content and updated it, making the old version no longer valid. Essentials adds new content, some of it very different, but outside of some powers being errata'd, and the change to sneak attack, etc, the old stuff is still valid. The PHB classes were not replaced by the Essential builds anymore than they were replaced by Martial Power builds.
There are legitimate questions that some people raise, such as whether or not there will be support for pre-Essential stuff going forward. Will the fighter, for example, ever get new encounter and daily powers to support their older builds, or will all new content be tinted by the lens of the Essentials mindset. However, the old content, even if it is not directly supported going forward, is still core to 4e and part of the post-Essentials game system. It hasn't been overwritten by 'fixed' versions of the classes, it has been complimented by more variations of the same class. Now, because of each build having different "parts", some of the old feats and the like are not compatible with the new builds ... then again, there has been build specific feats for a while now, that is nothing new.
The biggest "problem", from my point of view is that many of the Essential classes are not compatible (at the moment) with hybrid or multiclassing. However, it seems that the upcoming book which reprints the original builds for the Fighter, Rogue, Ranger, Warlord and Wizard will also contain that information.