The reason that there are so few edition discussions that focus on specifics/mechanics is that very few non-adopters (in my experience) have rejected 4E for specific/mechanical reasons ('I would play it but for the multi-classing rules', 'It's great but I can't bring myself to like minions'). They just don't like the taste. Asking them 'but what do you specifically dislike about the taste?' probably won't elicit any kind of constructive response ('I dunno... it's just... different' or the classic 'It just doesn't FEEL like D&D').
Actually there have been some pretty extensive reasons listed in numerous places, which revolve around actual game play or the books themselves.
The reason I consider it a step backwards is because of what roleplaying games grew out of and what is unique about them. We all know that RPGs grew out of tactical strategy games, where you were essentially pushing historical army units around on a board, battling other armies to see if you can out-general your opponent. One day Gygax and Arneson decided to apply the basics of that game to allow you to simulate a single character in a fantasy world. His inspirations were fantasy fiction, which I'm not going to cite because we all know the story.
So while RPGs included simulated combat as a core element, the tactical element was de-emphasized and it instead placed an emphasis on recreating your favorite fantasy stories. In short, it was intentionally about simulationism, not gamism.
Now don't get me wrong, I don't HATE 4E. It did some things right, such as making your BAB and saving throws based on 1/2 level + ability modifier and then spread character progression over 30 levels. This really takes care of the problem with 3.x being difficult to play at higher levels and it spreads out the playable levels well beyond what they are in 3.x. If they would have done that and then added in a few of the tweaks from the Star Wars SAGA edition, I think it probably would have worked.
Instead, I see 4E deficient in three key areas.
1. Gamism. The rules focus on combat only. The rules that allowed you to interact with the world in ways that don't involve combat were mostly stripped out of the system. On top of that, while I like the basic concept of powers, I think they got way too carried away with them, making it so that combat revolved around them. Why even bother to include special combat actions and feats if you're going to just build them into the class powers? Also, since your at-will powers automatically double weapon damage, why would you not use them? They're too good. You're never going to choose to not use them, which leads to combat being very repetitive. I prefer the method of having your regular attacks modified by feats. Then there's all the forced movement, and the counters indicating various combat conditions of varying durations. It appears to have been meant to make combat more dynamic, but to many, adds complication back into combat and makes it less fun. To me, this looks like the game is moving closer to its wargaming roots, which is something that I personally don't like. I feel that the right area for gamism and simulationism is somewhere in between 3.5 and True20. 4E is a fine miniatures game, and it's fun in its own right, but I have more fun with 3.5 for actual roleplaying.
2. Layout, white space, and wasted space. With the large font size, the reduced number of character races and classes, magic items, spells, and the amount of space in the books that is literally wasted, I can't help but compare it side to side with 3.5 and wonder what the reason was for excluding so much stuff. I'll avoid projecting motives on WotC, but I will say that I feel like I'm getting less game for more money. Sure, spend more money on more books and get the options you're missing, but I want a core that's a little more complete and more consistent with the past couple editions of D&D. The powers themselves are large, spread out, and colored in where they could be greatly condensed, saving space, which would provide room for some of the things they decided not to include. Yeah, I'm not a fan of powers in the first place, but those things are space wasters. Then the things they chose to cut in favor of the new things they chose to include is just puzzling.
This doesn't even begin to address the fact that combat grinds. There are plenty of tangible reasons people aren't enjoying it.