This is Flatly Untrue, tho the marketing department loves that people keep claiming this.
Wizards? Clerics? About 80% compatible. Major changes to class features make much of the previous non-power support completely worthless and much of the post-e support worthless to 4e classes.
Every other class that shares a name with a pre-e is more than 90% incompatible. Drastically different features, a completely different(and incompatible) power set than AEDU, role changes and changing even starting HP,HS, skills and profs.
No, they arent just new builds, they ARE new classes. Most of which are at least overpowered, have little or nothing to do with their supposed 'role'(every class is based on extra damage) and are extremely boring in their implementation.
...and all that is before we arrive at the braindead idea that is Item Rarity....
I generally disagree with what you've written, but I would just like to address a few points in particular:
1. While not all Essentials classes have the same at-will, encounter and daily structure for their attack powers, as far as I can tell, all Essentials classes have utility powers at 2nd, 6th, 10th, 16th and 22nd level, and in most cases, are allowed to choose one out of a list of possible powers. Hence, by even the strictest reading of when Essentials characters are able to choose a power from another (non-Essentials) source, they are generally able to make use of "classic" 4E utility powers by selecting one of them instead of the utility powers listed in the Essentials class descriptions. Given that a "classic" 4E character normally gets up to 13 powers from his class (2 at-will, up to 3 encounter, up to 3 daily, up to 5 utility), that's (IMO) a fair amount of compatibility when it comes to class powers.
2. While it is true that not all class-related support for "classic" 4E classes are suitable for Essentials classes, and vice-versa, this arises from differences in class abilities. Essentials knights and slayers are still fighters, and can choose and beenfit from "classic" 4E feats and other game elements that just have "fighter" as a prerequisite. In addition, the races have been largely unchanged by Essentials. Most of the time, an Essentials character can choose and benefit from "classic" 4E racial feats and other game elements.
3. It is not clear to me how the perceptions that Essentials classes are "overpowered", are "based on extra damage", or "have little or nothing to do with their supposed 'role'" were arrived at. It seems to me, at least, that the leaders still heal and buff, the defenders still punish enemies who do not attack them, and the controllers still shape the battlefield in various ways. Naturally, the strikers still deal damage, and I can only surmise that the over-representation of strikers in the Essentials classes could be one factor that has led to the perception that the Essentials classes are "based on extra damage". The other possibility could be the damage-increasing class features possessed by defenders such as the knight (fighter) and the cavalier (paladin). However, these are (IMO) simply a substitute for the encounter powers that would have been gained by their "classic" 4E counterparts. Such Essentials classes are effectively trading off the ability to choose a variety of encounter abilities for multiple uses of a single, more generally useful ability. In fact, it seems to me that such trade-offs are quite common in the Essentials classes. To be sure, they are different trade-offs from what has been found in the "classic" 4E classes, but (IMO) they are trade-offs nonetheless. Hence, I see no reason to believe that the Essentials classes are "overpowered".
4. The observation that the Essentials classes are generally simpler, require the player to make less choices, or have less varied powers, is quite valid. However, whether this is a "boring" bug or an ease-of-use feature really depends on what each individual wants out of the game. Similarly, opinions on the item rarity system are quite varied. I personally like the options provided by the Essentials classes and the magic item rarity system - one allows me to accomodate a wider variety of players in my games and the other give me a system to control the players' access to magic items if I want to do so, but as always, YMMV.