I agree with this completely. I have a campaign where I have carried characters through 3 editions of D&D. When 4e was released it was virtually impossible to convert characters.
I'm curious: what characters couldn't you convert?
If 3e is distinguished by anything (and it has many points of distinguishment), it is by the vast array of options it has. If you try to convert a 3e character to an AD&D character, you're going to find it extremely difficult except in a few limited instances.
4e doesn't have those options (yet), so it's no surprise that the Incarnate has no direct comparison in 4e. For the Wizard, there's a bunch of stuff in Arcane Power which will make conversion easier, but as the designers were trying to change the Wizard to make it less of the "I can do everything!" class, it's no surprise that the 3e Wizard will have options that the 4e Wizard will never have. Even so, I can convert my AD&D magic-user to a 4e Wizard and feel that I'm in the same ballpark. It helps that my signature spells were
fireball and
lightning bolt!
4e is as different from earlier D&D as any of the competitors.
Hmm. That's a very big - and inaccurate - generalisation. Would you really say that 4e is as different from 3e as, say, the White Wolf storyteller games are from 3e?
4e keeps the core of what every edition of D&D has used: Your level determines your attack bonus. You then roll a d20, which is then compared to a defensive score (AC), that inflicts damage in hit points. You fight at full effectiveness until your hit points run out, and then you fall unconscious or die.
The other part of "core D&D" is the concept of class/levels: you have a class which you gain levels in, and higher levels give you greater power.
Everything else is window dressing. Every edition has had different takes on what you add to those core concepts. Look at the vast number of 3e classes that try very different things: Incarnate, Binder, Warblade, Psion. Are they all D&D? Surely so - but they have very different takes on how things work.
I've been playing a bunch of 4e without using miniatures or a board; it works pretty much as well as 1e did so, which is to say, there are times when you need to trust the DM's view of where everyone is and what people can do. Roleplaying? A lot of that, and we have tools to aid it mechanically if we want to use them.
Cheers!