I'm not sure it's entirely a rules thing either. Ruleset matters less to me, but when you slaughter sacred cows willy nilly and redefine many of the game's concepts, including using classic names for very different creatures, that's where the fundamental disconnect is. I wouldn't discount the flavor and fluff changes as part of the reception, alongside the rules.
Are you actually claiming that altering too much of the flavour/fluff means the game doesn't qualify as D&D any more? Because I'm damn certain that half the D&D audience plays a homebrew setting, and they aren't doing that because they love the default fluff and setting(s). And I'm equally sure that they still think they're playing D&D. An awful lot of published settings have been making burger of sacred cows, as well. And they're still D&D. So if it's not a rules thing, then you're going to have to explain just what it's acceptable to change about the flavour.
But that's what we've got _now_. A shoe labelled "D&D" that doesn't fit a lot of people who it once did.
A thousand times this! It seems that they realize this somewhat since Essentials seems to be an abrupt backpedaling to try and win back many that passed on 4E. Unfortunately, it seems to be too little, too late. They now have Pathfinder and retroclones to compete with. It seems they were doomed once they decided to make a complete break with many traditions of the game. D&D has strong name recognition but not strong enough to sell a game that bears little resemblance to the D&D that many fans cut their RPG teeth on.
I was 47 earlier this year. I've played D&D since the 1970s. And you're describing 3rd edition as well when you talk of a shoe that doesn't fit.