The rules are not the same for npcs as they are for pcs.
The difficulty of a task is based not on the task but on the character level.
All wizards get better at climbing.
The black knights and pirate captains ACs are both based first on challenge level, not what they are.
Square fireballs and 1-1-1 diagonals.
The list goes on and on.
And I understand the point of the changes.
And it really isn't the changes themselves so much as the driving philosophy that led to them and colors the entire game experience.
And I don't claim they ruin anything, but they detract a bit at a time. In the end it is B-.
I haven't read 4e rules. What BryonD mentions as a short list of faults, some I agree with, some are meh.
Each edition had people in the prior edition thinking it was crap, 4e had the most dramatic changes to my eye. I could have taken the spirit of the 4e ideas, and not as dramatically changed the game. I think that bugs a lot of people. Moreso than the "they changed my game" reaction we normally get at each edition.
Since I can have and have told a variety of kinds of stories in 2e and in 3e, I'm certain I could do those same stories in 4e, or some other RPG.
To the OT then, the 4e experience may be the most dramatically different than the others. The prior editions had mostly the same classes, same races, same spells, same combat handling. 4e took out a bunch of classes (Bard?) added new ones, and changed what everybody could do, and when people talk about it, and "roles" and "Job" it just doesn't sound like the same game. Character ideas that would have worked in 1-3e just don't work in 4e (barring the new books adding stuff).
I think the complaint is, the differences in this revision are greater than the differences in all the other versions. In a way, all the people who complained about the changes in prior editions look like whiny nit-pickers. Because 4e made more changes in total to the game, than the others did.
I'm not gonna say 4e isn't D&D. It's got it printed on the cover by the owner of the copyright. I guess it is.