My dislike of Essentials was mostly targeted at specific parts and philosophy issues, rather than the whole of it. Some parts of Essentials were quite good, like the improved Expertise feats (which were actually interesting and involved choices) and the much, MUCH better MM3/MV math.
I emphatically DID NOT like the "alternate roles within a single class" concept of subclasses. It added a ton of weird jank for very minimal benefit. "Slayer," "Blackguard," etc. could easily have just been presented as standalone classes (call Slayer "Myrmidon" maybe, for a classic vibe). Several other Essentials subclasses were also just...really really not good, some of the very few outright weak options in the game. Binder and Bladesinger primarily. Vampire was a tough call, because it could be made to work, but it required rather a lot of optimization to perform, which ran counter to 4e's prior commitment to being at least okay at whatever a class's core function was supposed to be.
I also, as others mentioned earlier, disliked that it felt like openly caving to the vocal minority of critics. It felt like the first round of repudiation, to be followed up by the Next Playtest, where I got the "expectation delay" parade.*
Finally...it just felt like they didn't really know what they were doing. For example, the Sentinel Druid has seasonal sub-sub classes, Spring and Summer. Someone--IIRC Mike Mearls--responded with utter incredulity when players asked when we might see Autumn and Winter Sentinels. Instead of doing stuff like that, they hyperfocused on spellcasters, particularly Arcane ones.
Now, again, I don't want to give the impression that I dislike EVERYTHING about Essentials. It did plenty of good things too. Beyond the two examples I gave above, I love the Elementalist Sorcerer, because it gave us, for the first and thus far only time, a TRULY SIMPLE spellcaster, literally AS simple as the truly-simple Slayer Fighter. That's great! I would absolutely love it if 5e took a leaf out of 4e Essentials' book and made an Elementalist Sorcerer that was of comparable simplicity to most Fighter subclasses in 5e.
*That is, "oh don't get upset, they'll give you 4e options later in the playtest." When nothing shows up for over a year, "oh don't get upset, they'll include something in the final packet." When nothing shows up in that packet, "oh, don't get upset, they'll add options to the first book." When nothing shows up there, "oh don't get upset, give it another two years, you'll get something by then." And then finally, "Be happy you got anything at all, they could've cut dragonborn entirely." I was literally told, for years, that I wasn't allowed to be upset--and then, finally, when I'd cleared every last hurdle, I was told I wasn't allowed to be upset because at least I got something out of it.