Tony Vargas
Legend
The Essentials Warpriest is closest to being a 'pre-built' 4e class (WIS Cleric), though it has plenty of it's own powers, so you could't just mimic it with a pre-Essentials Cleric. You could, however, take a Warpriest and trade out it's pre-chosen powers to get close to a pre-Essentials Cleric, and a WIS Cleric that took Warpriest powers could be very close ot a warpriest. Warpriest powers are pre-chosen based on a single player choice of 'Domain.' It's a good apraoch, it makes chargen and level up easier, without flatly taking away options as was done with the fighter. If there were a similar STR build in Essentials, the Cleric would even be 'fully supported' by Essentials.I haven't gone too deeply into Essentials, but I'm wondering if an Essentials fighter, for example, is mainly just a pre-built 4E fighter. They both basically do the same thing, but the standard 4E fighter has more options, more maneuvers (powers) to choose from. An interesting question would be: Could we build or "mimic" an Essentials character with Character Builder and the existing 4E rules?
The Mage is also pretty miscible with the 4e Wizard, they can swap out powers and such. But there are some differences that seem to amount to the Mage just plain being a bit better.
The Knight could be closely mimicked with a Fighter. For instance, a Knight might have one stance that pushes the target and lets him move into the target's square (Hammer Hands) and another that lets him do damage to a second target when hitting a primary (Cleaving). A PH1 Fighter could do the same things with Tide of Iron and Cleave at-will attack powers. Similarly, the Knight punishes adjacent enemies for attacking his friends, and the Fighter does something similar by 'marking' enemies he attacks. So, functionally, they're quite similar. An Orc hit by Hammer Hands on one round and Tide of Iron the next wouldn't be able to tell you which was the Knight and which was the Fighter from that, alone.

The Knight and Slayer certainly /could/ have been done like Warpriest, with exploits pre-selected based on weapon choice, for instance.