I'm a 4e fan, ran games with it for five-ish years, and hope to run games in it again one day. People point to one thing or another, trying to emulate mmos or making everyone essentially a wizard, and what have you. I don't think any of that was really the problem, or that they were necessarily even true.

4e suffered from three things mainly that really killed it:

1. it fundamentally misunderstood what people liked about or wanted from dnd. 4e is almost a pure dungeon crawler and is perfect for an older style of game which is chiefly played in dungeons without too much time in the overworked, and most of the adventures released for 4e reflected this past style as well. The demographic for that style of game was exceedingly small at the time of its release, and has only gotten smaller.

2. 4e is completely drowning in bloat. Item bloat, power bloat, feat bloat - after fifth level, your character sheet becomes so unwieldy that it slows combat to a crawl all by itself. And 4e combat is already slow. They tried to mitigate this with cards and online tools, but the cards were just as unwieldy and the online tools never really got off the ground. The character creator app was great, but everything else never made it past development. (Due to a murder-suicide thing, actually, but that's another story)

3. 4e did not have an OGL, instead using the GSL which restricted a lot of the rights of third party creators. As a result, 4e had virtually no third-party support, which means that all of the companies that made content for 2e and 3/3.5 had to start making content for other games. Most famously, this resulted in the creation of Pathfinder, transforming Paizo from a company that published dnd-centered content to arguably its chief competitor.

A lack of third party materials plus the poor cost-to-benefit ratio of Wizards producing adventure modules on its own meant that 4e had a lineup of books that were largely core rules expansions or setting splatbooks(both expensive products for the consumer), but very few adventures. That translates to poor shelf presence, less actual play time, poor word of mouth, etc. Before you know it, the only people that ever talk about 4e are the people who didn't like it, since most other people didn't even play it.

Some of the stuff that people didn't like about 4e: the powers system, the lack of social interaction rules, the skill challenge minigame, movement to a hard grid-based system, and game mechanic being expressed in doyalist terms. They also felt that the games' reliance on keywords was video game-ish and added a limiting amount of granularity.

The powers system is flawed, yes. Its one of the main causes of character sheet bloat. But the complaint that all classes play the same because of it is not really founded. Every class gets powers and they are all structure similarly, but that's really the result of a well designed and highly functional system. The actual content of the powers is pretty diverse, leading to unique playstyles across all the core classes and amazing moments of synergy.

I have yet to find an edition of DnD with great social interaction rules. I feel like people just throw this one on there.

The skill challenge has been re-evaluated in recent years, and most tables use some variation on this mechanic in their 5e games.

The grid system used in 4e is still used in 5e to this day - they just changed the notation from squares to feet. This is why we don't have to do any taxing diagonal movement math in 5e. It seems this one change eliminated most peoples problems with it and showed them how they could still use it for theatre of the mind style play.

The doyalist design of the books, combined with all the keywords, is actually pretty good design. DMing a 4e game is a dream for these reasons. And most ttrpgs employ a more doyalist design philosophy anyway - but 3.5 didn't. Which is why it felt bad, I think, to people who tried converting over. Gotta say I agree with the granularity issue, though.

One of the ironic things is, people don't realize just how much of 4e's DNA is in 5e. 5e is sort of just a bunch of good ideas from 3.5 and 4e shoved together, their respective bloat removed, and trimmed up with bounded accuracy.

This turned into something longer and more ranting than I originally intended, but I'll leave it as is and hope you find value in it.