I don't think it's possible to escape the fact that 4e is quite different from every edition that came before - both for good and for ill. Some of the changes I like, some I don't; but it's still different.
And yes, I do have a broad base of experience outside of 3e, if such be needed.
See, this is where I kind of get bogged down.
Mechanically, 4e is far closer to 3e than 3e is to, say, Basic/Expert D&D. It's actually probably closer, purely mechanically, than 3e is to 2e.
Note, the flavor changes are bloody huge and you'll get no disagreement from me here on that. But, that being said, you can point to a LOT of the flavor of 4e being informed by Basic/Expert and 1e D&D. The idea that artifacts are weird and wonderous, rather than just really powerful magic items that PC's can't make, for example. Disconnecting the rules for PC construction from NPC construction is purely pre-3e D&D. That sort of thing.
So, I really do have an issue with people talking about how different 4e really is.
I mean, you can sit a 3e player down at a 4e table, hand him a character sheet and he'll be able to read the character sheet and understand most of it. About the only difference from the player's perspective is encounter and daily powers. That's the only mechanical difference.
If you sit a 2e player down at a 3e table, hand him a character sheet, he can't even begin to parse the sheet. Stats are handled entirely differently, saving throws are entirely reworked, classes and levels work differently, everything on that sheet is different. Never mind that as soon as combat starts, the 2e player would have zero idea how gridded combat works. The 3e player should immediately understand how 4e combat works. The rules are largely the same.
I think one of the biggest differences though is in the voice of the books. 3e core books were as "voiceless" as possible. They presumed that you already knew how to run a game and generally handed you lots of intricate detail, but, very little advice. 4e presumed you didn't know how to run a game and, like 1e, had a very strong voice on how to run a successful game.
I think that strong voice is primarily what makes people grind their teeth.