I think the reason many non-4e players are reacting so strongly is because they really don't understand 4e design. It continues to be expansion-based design in the tradition of Cosmic Encounters and Magic: there is a fairly trim set of rules, and each class is a nicely bundled set of rules and options that expands on that core set. Adding another bundle of rules doesn't change the underlying rules any more than adding a new keyword fundamentally changes Magic.
Depending upon how you view "a fairly trim set of rules, and each class is a nicely bundled set of rules and options that expands on that core set", I would argue that this is the model that
all D&D editions have used: "This is the core. Here are some expansions".
One doesn't have to grok 4e to understand this. One merely has to have some experience with D&D in any of its forms.
You didn't have to be hardcore to not use some of the Player's Options books. They made for a substantially modified game, easily as big a shift as going from 2e to 3e but without the fanfare and support of a full new edition.
This, however, is the issue IMHO. A lot of people now view the Player's Option books as a kind of 2.5.
The difference between an edition, or a "half-edition", and an expansion of a current edition, seems to be whether or not classes or basic concepts are rewritten. Player's Option was touted as an expansion that could be used alongside your current rules. It gave optional new forms of classes, rather than replace the existing forms. Yet, hindsight is now 20/20 -- it was to 2e what 3.5 was to 3e.
Likewise, you could certainly use elements of 3.5 with 3.0 "without even a burp". That was easy to do. For instance, you could run 3.0, but use the 3.5 ranger. I did this, and know many others who did the same.
Heck, you can use 1e materials with 2e without too much problem. Hardly a burp, if that. Indeed, the 2e expanded class books
reprinted the classes from 1e. I certainly ran Basic and Expert modules in 1e "without even a burp". How, then, does 1e or 2e qualify as a new edition?
No, I think we have reached a "Book of Nine Swords" moment, and hindsight will later be clear enough when 5e comes that what the Essentials line represents in the earliest foray in 5e design. The cool thing about such a prediction is that, given enough time, there is a fair chance of either finding out that I am right, or that I am wrong. And I am fine with waiting until then.
YMMV.
RC