I feel like the things that were good about essentials were incidentally good, and have to do with compatibility with the base game-- for example, having the 'low choices' variants of the classes alongside the original ones was a nice variety for some of my players who wanted a less fuss approach to character building, or allowing 4e characters to loot Essentials options here and there was nice, but the actual direction of the material was made tolerable in the context of 4e being matureish, a theoretical essentials Knight/Slayer-only fighter is a lot worse without the Weaponmaster right there for people who think they're too boring.
Getting the Hexblade subclass was nice for the warlock, and the Elementalist was a neat concept for the sorcerer, but both rely on already having the full 4e warlock and sorcerer to fall back on.
The Essentials Executioner Assassin, while nominally stronger than the O-Assassin, was even less distinct from an assassin built out of a rogue.
The Monster Vault was lovely in the sense that it represented an opportunity to apply MM3 math to MM1 monsters.