These claims about "difference" are contentious. For instance, I've played plenty of B/X, and more than plenty of AD&D, and I don't find 4e to be a "drastic departure" that is "completely different from any previous edition". Like those earlier editions it has fighters, clerics, thieves and magic-users (wizards). It has a default story arc that begins with goblins and bandits and ends with demons and devils and their overlords on other planes. It has to hit rolls and hit points.
It expands the imposition of effects, as a consequence of combat success, from spell casting to martial combat but that is not very confusing, and not unprecedented either: the Wilderness Survival Guide had rules for bleeding wounds, and one of the Appendices in Unearthed Arcana had rules for stunning enemies by hitting them over the head with a chair.
I don't have any experience with 3.5, but my limited experience with 3E suggests that 4e is no more different from it than 3E is from 2nd ed AD&D.
The labelling of things that have always been part of D&D - like different sorts of abilities with different recharge periods - as at-will, encounter or daily powers doesn't seem to me any very big change, no bigger than (for instance) 3E turning non-weapon proficiencies into a mixture of skills and feats. It's a change in label, but not in play.
I'm pretty sure that I'm not the only person, even the only person in this thread, who sees the issue of difference this way. Which means that I don't see how difference is a very useful basis for explaining what happened with 4e and Paizo. You would need to identify some other factor which explains why some people think 4e is different from what came before, and why others don't.
And that explanation is unlikely to be that one group is more insightful than the other, or that one group has a better grasp than the other on the essence of D&D.
Well, I'm one of their customers and they didn't take the position that I don't matter. They published material that I wanted to purchase, and I did so.
"Poorly designed", here, seems to be treated as synonymous with "not popular with me and those who want the same things as me". Which is not the same as "D&D players" or "WotC customers".
The comments about legendary monsters are also interesting, because when I look at them I see them as a clear development of a 4e idea, the solo monster with mechanical abilities to try and handle the action economy and also cope with action denial. This is building on one of the more distinctive, and better designed, aspects of 4e, namely, attention to the action economy.
I also think it's interesting that the solution to ongoing problems with 4e solo design has been to go more metagame - with legendary resistance and legendary actions - than nearly any element of 4e monster design. It makes me somewhat curious as to why we haven't seen those who attack "dissociated mechanics" attacking legendary resistance and legendary actions. But I think the answer to that question belongs to the domain of marketing, rather than of game design.
For me, the 15 minute adventuring day began in AD&D, when casters nova-ed their spells and wanted to rest. In six years of 4e play I have not seen any problems with the 15 minute day, because dailies are only a very modest component of PC effectiveness.
That is a departure from earlier versions of D&D, where dailies - in the form of spells - are a very significant component of PC effectiveness.
Another interesting feature of 4e that 5e seems to have picked up on and developed is the ritual system, decoupling effectiveness in a whole range of utility situations from the expenditure of daily resources.
