Good / Bad - as always, is a thing decided by the beholder.

What Essentials did (as objectively as I can):

(A) create some new forms for some classes
Some [martial] classes were vastly simplified and lost [Daily] powers - the purpose... well, that can go from : an interesting take on making the classes simple and more stream-lined for players wanting fewer options when building their characters and at the table during play... all the way to : appease the haters by making [martial] strictly inferior to [arcane]

Most [caster] classes kept the same level of complexity, and some even got buffs! (that were not required, like, at all) BUT! later on, we got the Element Sorceror which is a very straight-forward caster (~same complexity level as Essential [martial]).

All these new classes worked very well together and with past ones IF you stayed in the Heroic Tier. After that, some cracks start to show up in many cases.

General player agency was brought down : particularly in the form of Rituals being removed from assumed play

Some of the BEST monster books EVER : Threats to the Nentir Vale isn't worth 200$+ on eBay for nothin' - same for DM box... Wish I'd bought two of each!

Some much, much better adventure design : Reaver's of Harkenwold is a top tier adventure by any metric

Create a good deal of involuntary(?) "chaff" : they introduced many options that were, straight-up better than anything that had come before. This had the side effect of making many previous options "traps"

Introduced a completely new Magic Item acquisition/management paradigm
Before, all items were available to be crafted (depending on DM setting parameters - a point often glossed over...) and or sold/bought with regards to their power level as the only variable.

Players had a number of available activation of [Daily] powers, no matter how many items they possessed.

Most items were fairly narrow in their power offerings - most items did the one thing.

With essentials : a new system of "rarity" was introduced where only [Common] could be crafted or bought. So a new metric, orthogonal to Level. (Full disclosure: I was not happy with this...)

Selling / disenchanting items gave a different return based on rarity.

Pretty much everything from before Essentials became [Uncommon] - which can pose some problems...

You can activate as many things as you possess, there is no longer any character-based limit.

[Rare] items are often much more powerful (at the same level) as [Uncommon], and the same for [Common] - but not always (which leads to problems...)

Important note
Any and all of the changes brought had positive and negative effects. On the whole, I like what they did for many classes, loved the adventures, really liked the items they added, hated the rarity system (like... hated that), greatly disliked the removal of Rituals.

Also, I disliked the new presentation - so much white space! SO MUCH! And the books... Augh!