What maintains the balance for Slayers is something like this...
Mighty Slayer
Slayer Weapon Specialization
Inexorable Slayer
Armored Mobility
Constantly improving Power Strike
Constantly improving striker bonus damage
And how does that work, exactly?
Clearly, improved power strike and weapon specialization keep the Slayer's Power Strike roughly on par with 'E' part of AEDU classes. Additional uses of power strike are accumulated about as fast as additoinal Encounter attacks, and, the way it upgrades very aproximately matches the way AEDU classes turn in low level Encounters for higher level ones. But, unless it's somehow demonstrably superior to more varied Encounter attack powers, it can't contribute to compensating for the lack of dailies.
Mighty slayer is the improving striker damage bonus, other strikers do have damage bonuses - the Sorcerer's, I believe, is very strongly comparable to the Slayers, isn't it. A secondary stat adding directly to damage rolls? So it's the amount over the secondary stat that's contributing to balancing out dailies...
That tops out at +8 damage.
Inexorable Slayer is a +1 saving throws.
Armored mobility is DR vs OAs.
The slayer also eventually takes no movement penalty for armor.
Two of those are about equivalent to feats, armored mobility is more unusual, but the net effect (worry a lot less about OAs) is basically comparable to the artful dodgers AC bonus vs OAs.
So, really, it's down to +8 damage, which is pretty sweet, I have to admit, especially for a striker. And, the Slayer's basic-attack emphasis gives him yet more easily obtained damage bonuses, and the ability to exploit charge enhancements, OA enhancement, and granted basic attacks. OK, it mostly comes down to using Bracers of Mighty Striking over Iron Armbands of Power, but it's nice.
+8 is actually looking good. Let's compare it to the damage potential - just the damage potential, none of the other rage effects - of the top-level Barbarian's dailies. The Barbarian can potentially have 3 7[W] dialies (one from Frenzied Berserker) and a 4[W]. I'm going to assume a Mordenkrad, because it just makes the math really easy - at 2d6 Brutal 1, it averages 8 damage. (see, easy) At-wills do 2[W] at top level, so the Barbarian is getting 17 [W] out of his dailies. All the Slayer has to do is attack 17 times (we'll ignore that the daily might have a miss line) in the course of a /day/ and he's matched that. In a 4 encounter day, that just requires 4-round encounters. That's not hard at all.
Damn. I'm mildly dissapointed. I was expecting the tipping point to be a little less on-the-nose. Clearly, someone at WotC - contrary to CharOp opinion - does have math skills.
So, imbalance occurs when you deviate in either direction: If you have a one-encounter day - and it doesn't go 17 rounds - the Barbarian comes out ahead. If it's critical to bring down an enemy quickly in an 'alpha strike,' and combats are very short, the Barbarian comes out ahead. If you have a grueling 8-encounter day, the Slayer pulls ahead. If you tend towards many-round brutal 'grinds,' the Slayer pulls ahead.
Since damage is central to the Striker roll, it's an easy analysis - which is why I asked for the Slayer, not the Knight. We could try to account for everything rages do besides damage, or every feature the slayer gets as it levels, but there's no point. On the basic Slayer function, we've isolated a point where the Slayer theoretically balances with a conceptually comparable AEDU Striker.
Whether, I got the estimate or math right, and whether every other non-AEDU balances out at quite the same point as every other AEDU class, I'm will to bet that WotC did their best to get them all balancing around the same 4-kinda-short-encounters point. Encounters is using 4 kinda-easy encounters a day. I believe the DM kit recomends 3-5 encounters/day, and the new higher-damage monsters edge the game towards slightly shorter combats.
So, we have balance at something like a 16 round (one-and-two-third minute) Day.
Deviate from that point consistently in both directions, in the same magnitude, and you retain a sort of overall balance of contrasting imbalances. Deviate from it consistently in one direction or the other, and you have a clearly imbalanced state.
Contrast that to 4e, where you do not introduce imbalances by deviating from a given number of encounters per day and rounds per encounter.
That's what I mean about Essentials degrading class balance, in the same way, but to a lesser degree as earlier eds.
Like I said, feel free to be fine with that.