My first ever 4E PC was a cleric, played on June 6 2008. The char I currently run is a PH 2 druid.
In the past months I played 1 session each as an essentials cleric and an essentials druid, both at level 5 (me meaning my PC). It was a snooze fest. At the very same level my PH 2 druid got 6 at-wills (8 if you factor in his hedge wizard gloves - he's a druid on the run masquerading as wizard). My essentials druid at the same level had 1 at will.
ONE.
And oh, he also had his two basic attacks, melee and ranged.
The cleric was similarly boring. I had nothing interesting to do. This really irked me. I love the 4.0 cleric. Next to the fighter it's one of the classes that 4.0 got really right, and right out of the gate. That
Essentials was the stated excuse to
rewrite the PH 1 cleric into something vastly less interesting didn't exactly endear me to Essentials either; though I wish they'd just left it alone. Nowadays I can't even point friends who only got DDI to the char builder and say 'hey look at that, that's a class I really love and think well designed'. The class I like is gone from DDI.
So yeah, essentials druid and cleric. It's not just the slayer or the knight, it's really a lot more pervasive. There's also a lot of goodness in individual powers - e.g. getting rid of crit expanding stuff; having multiple options in 1 at-will (Essentials ranger, cf. PH 3 psion) - but the point is, this comes to little if the overall class design doesn't come well together. And in my limited experience, it just doesn't.
And it's not just the classes in Essentials. It's the races too. I loved the goodies that humans and half elves originally got - precisely because they enhance your at-wills. Now their racial bonuses are... not that great, if you ask me.
The one thing I'll give Essentials is flavour. I really liked some of the Essentials druid's class abilities. I think the opening paragraph to the Class Compendium on the rogue (which is the PH 1 rogue rewrite) is great; much greater than anything in PH 1. But such flavour text is not relevant in play, or at least it quickly fades away. And the druid's extra stuff - well, stuff like 'talk to animals at will, ask 3 questions' was already part of our campaigns, druid can speak to animals, yes of course. Can't really see how that type of thing can make up for a
lack of meaningful choice in the power array. Might as well be allowed to wear green garments by trading away 5 at-wills.
I don't begrudge people who use essentials classes (some of my friends do), or those who run essentials-only campaigns. But personally I'd see it as a significant bump on my fun playing 4th edition if my GM said to me 'you got to play an essentials character'.