It's tough to discuss 4E without getting into edition wars ... but to me it always felt like a very different game. Not necessarily a bad game, but more of a card based tactical war game with too much going on at higher levels which caused gameplay to bog down.
I enjoyed it well enough at lower levels and the design seemed simple but the interaction of powers/interrupts/interrupts of interrupts at high level had me looking for a new game when 5E came out.
I've always said that 4e is one of the best tabletop tactical squad-based combat games I've ever played. And I still believe that to be true today. It's a fantastic game in that genre. The addition of a few fringe mechanics like skills was enough to turn it into a fully fledged RPG. However, it never felt like D&D while I was playing it, and we ran into problems running it as a TTRPG.
I can even pin down the exact session that we all knew we were done with 4e. It was our third(ish) campaign in late 2009 to early 2010. There were seven PCs. I couldn't even tell you what classes the other PCs were, but I was playing a Fighter. We were low to mid level Paragon. Level 13 or 14. About half the PCs had enough system mastery to create a build, and the rest had a good idea how combat should work and what roles they should fill. We had the Character Builder and a nice laser printer, so we should had little difficulty with the management of characters and abilities. At this point, one or two players had already voiced that they were struggling to enjoy the game; they just weren't having as much fun as 3.x or AD&D.
I don't know where our DM got the module -- it was something online that had been produced in the first few months the game had been available -- but the module had already been very wonky in the past. This was an edition where the DM was rather discouraged from altering things on the fly and it led to very strange outcomes. This was the same module where you had a DC X Diplomacy check to convince a Duke to let you help to rescue his daughter, and if you failed you went on a subquest to get help and evidence that you could succeed. When you came back with help and evidence (a few levels later) the DC of the Diplomacy check to convince the Duke had gone
up. Even knowing that DCs aren't fixed in 4e, it felt like we spent a bunch of time on a side quest to make it
harder to convince a Duke that his only daughter was worth saving. (And, no, he wasn't secretly trying to off her. We asked.) Oh, and by the way, if you failed this second check then you were stuck at a dead end. You needed the Duke's help, and he wouldn't give it if you failed the second time with the evidence. There was no alternative given to move forward. Yes, obviously the evidence should have automatically succeeded, but the DM hadn't read the second encounter all the way through to realize that. This was also the module where you had to complete a Skill Challenge to
get to a certain dungeon, and if you failed you took some kind of soft damage that basically meant once you started failing you were extremely likely to continue to fail. We only survived through some extremely lucky die rolling by PCs that should not have succeeded. Just very poorly put together, so a significant amount of blame for what happened next can be laid at the feet of the module author.
Anyways, back to our session of doom. About 30 minutes into the session, we began The Encounter. There were about 10 NPCs in a room with a lot of stairs and some partially submerged sections. A sewer, basically. Most NPCs were some kind of undead mummy things, while the rest were some necrotic or earth based construct, fiend, or elemental. Most of the NPCs had auras. Some of the auras did necrotic/poison damage. Others boosted necrotic/poison damage. Others granted resistance to radiant damage or everything except necrotic/poison damage (and all the NPCs were immune to necrotic/poison damage). I think some of them healed when they took necrotic/poison damage or had some kind of natural healing. Worse, least three of the PCs had spells that could create ongoing areas of effect, and every PC had reactions with some being at-will. This combat took forever. Four hours in and most of the players had checked out. It's not that it was a difficult encounter, it was that there was
so much bookkeeping for what was going on with all the overlapping auras and so many reactions going on that the game moved too slowly. In the end, we spent the entire 6 hour session playing one encounter
and we did not finish. We stopped because the DM was exhausted. The next session, we finished the encounter in the first hour, and got to the next town or safe area and ended early.
The session after that, one of the guys said, "Hey, I kind of want to run a 3e game," and everybody jumped at it. Nobody looked back.
I remember having a discussion with some of the guys later, and the overall feeling was that the only way to run that kind of encounter effectively would be to run it like a business meeting and really get a lock on pacing a being ready to act on your turn. Give every player responsibilities for tracking something, etc. The problem with that is that it would make playing D&D feel like attending a business meeting. Business meetings are about as fun as getting a cavity filled without all the entertainment of the drill and amusing ceiling posters. Nobody wanted to play 4e
that badly.