I don't think anyone can say with a straight face that 4e wasn't a failure, for a variety of reasons (I mean, a game with those sales would have been a roaring success for any other company, but not for Wizards). I would chalk the reasons up to:Heh, yeah there is.
I'm not a huge fan, nor a huge hater, of 4E . . . but I've noticed over the past few months an uptick in folks talking about 4E, talking about playing 4E today, and using 4E design choices to inform current game design.
I mean, it's nowhere near as huge or influential as the OSR movement, but yes, 4E is coming back for a segment of our hobby.
- Class homogenization, particularly in the core books. Even if the classes do different things, they all use the same structure of at will/encounter/daily powers.
- The focus on powers also meant that each class took up a lot of page count, and much of that page count was focused on combat-related things. This is to some degree a perception issue: other editions have had many classes saying they cast spells of a certain type and then refer to an appendix for those spells. This makes the class description itself pretty lean, by off-loading the page count elsewhere.
- Powers being combat-focused also gave the impression that non-combat magic was gone. It was not, but had been moved to the realm of Rituals which could in theory be accessed by anyone with the right skill and feats. This actually improved the access to non-combat magic by not limiting it to particular classes as well as not taking up daily resources, at the cost of making it less convenient (taking time and costing components to use). Again, a perception/presentation issue.
- Things people felt were important (metallic dragons, many giants, some classes and races, etc) removed from the core books. This together with new material added made people feel these things were getting replaced, even if they got added back in with later books.
- Changes to the assumed lore, with a very vague assumed setting that simultaneously stole some things from classic lore (Vecna, Tharizdun, mention of famous dungeons, etc.) and made up its own things (Dawn War, the fallen Dragonborn and Tiefling empires).
- Nuking the Forgotten Realms, essentially making a new setting with the same name as the old, in order for it to absorb all the new lore.
- Having a system that purports to be highly balanced, while actually being off regarding math (skill challenges in the core were almost impossible, monster defenses increasing faster than PC offenses, monsters having too many hp).
- An introductory adventure that would have been a fairly bad adventure for earlier editions, and was really bad for the new one because it played up its weaknesses and didn't show off its strengths.
That said, even if 4e as a whole was a failure, it had many good parts and what I think is happening is that these parts are being appreciated more. Many people appreciate having martial options beyond "I hit it with my axe". Positioning and conditions mattering means there's design space for abilities that apply and/or leverage those. Having a mix of encounter and daily powers is pretty cool, letting you both have "Is this the round in which I use my good thing?" and "Is this the fight in which I use my AWESOME thing?". If you like attrition-based adventure design, healing surges are a great way of handling it, because they let you make individual combats feel dangerous, then allow recovery, but still have an overall attrition that doesn't depend on the casters' spells – plus, they in combination with the Leader role, remove the need for clerics. And monsters have a fair amount (but not so many things become hard to deal with) of cool abilities themselves, in addition to just dealing X damage.