You lost me. Sentinel and Slayer are not subclasses, they are standard essentials classes (from what I can tell in the Compendium), and there is no "Sentinel Druid" (Compendium again)
This is incorrect. They are "essentials classes" (and have any comonality at all) only in that they are both printed in the Essentials books. But there's nothing particularly special about the Essentials books except that it introduces the non-ADEU subclasses, and the classes introduced in that book are all over the map in terms of how they operate.
A Sentinel is a druid, and can take most Druid powers (exceptions include only encounter powers and wild shape powers). A human Sentinel can easily be as good a ranged controller as a normal druid while still acting fully as a leader. They are a very wide subclass.
A slayer is a fighter -- but, lacking at wills, dailies, and encounter powers, is very limited subclass. The same for Knight.
By the same token, a mage is a different subclass of Wizard than the original wizard, now titled "arcanist", but there's really nothing "essentials-style" to a mage at all; they're a totally normal subclass with some different base class features and one fewer implement proficiency.
Functionally, there are two tech changes that were introduced in Essentails:
1. Classes are now defined as Class(Subclass), where Class tags powers and feats, and Subclass determines the range of class features available and the class substructure. Presuming that Wizards eventually (as they seem like to do) recodes all existing classes in this paradigm, Earlier classes will become subclasses of their overall class; we already know several of the names in question (Fighter->Weaponmaster, Cleric->Templar, Wizard->Arcanist) but not all of them. This doesn't necessarily involve a change in mechanics; the Mage doesn't include a mechanics change at all, the Templar is no more limited than the PH1 Warlock, and the vast majority of other new subclasses are only limited by restricted at-will choice (pretty normal, and avoidable by being Human as it was for the Warlock) and/or limited or no choice in terms of encounter powers.
2. The introduction of classes with highly variant structure; from the (non-essentials) no-power-choice Vampire to the Stance+U Slayer. Particularly of interest here are the no-daily classes, as they have only a single daily resource (surges/hp) instead of the dual sets of daily resources other classes here.
It sounds to me like you're saying you don't like #2, or rather don't like them mixed with some other classes. That's off topic--particularly since I'm talking about
#1 that the tech now allows for different subclasses (call them different classes if you like--though they can share powers, so not so much) that share a theme and overall means of operating, and thus are the same class. By creating a new class (and here, I think the Runepriest and Seeker were mistakes) or rather sublcass of an existing class, you get access to a pile of useful material that immediately makes your class useful directly along its flavor lines, rather than having to...not simply build a new thing from scratch, but having to duplicate material that has already been printed in order to fill your new class out in more or less the same lines as the old one.
Rechan: I agree that a ranged defender is doable -- but either you need to explicitly expose them despite operating at range (thus range gives them target choice, not invulnerability), or you need to factor their ability to operate at range, get behind cover, and sneak around into their defenses when balancing them.
So, a ranged defender might have wizard HP and defenses, for instance, but have the ability to operate from a significant distance, with an extra defense against ranged/area attacks (maybe a NAD bonus, or damage reduction vs non-melee). Opponents would have to make the same choice about "do I target the defender with a high chance of failure, or get punished for ignoring her" they do for, say, a fighter -- but instead of the thing providing a high chance of failure being great HP and defenses, it would be the fact that the defender is 10 squares away and hiding between a rock; the overall chances of taking her down are similar because she's squishy, but her defenses are the defenses of a ranged attacker, not of a melee character. [one one level, this is a word trick, as the above feels more like a specialized controller rather than a defender who takes hits. But on another level it makes perfect sense. It doesn't matter; you can play terminology games all you want and I won't care; the long and short of it is that one could make a balanced and interesting character this way].
Another approach would be a "stealth defender" -- probably one who would operate ducking in and out of melee (and probably also be very squishiy). I'd say that you could build one out of an Assassin, but the "Assassin" class just can't really lend itself to anything except a striker; to make such a class a sibling of the Assassin, you'd more or less have to make the Assassin a subclass of a larger concept covering the idea; Night Shadow or somethng. So how about making it a new Rogue; a Rogue doesn't technically -have- to be a striker, thematically. Call it Shadow Dancer [yeah, I know, that's a HoS paragon path. But Wizards doesn't shy away from name conflicts, I don't see why I should]. A Shadow Dancer would be a defender rogue, with rogue hp and defenses (so decent, but not great), but with good stealth abilities and the ability to defend at medium ranges. Marking mechanics are overdone, and we've seen the problems with marking auras (plus they don't work for ranged defenders at all), so lets say SD should use a "zone of control" mechanic that represents their attention -- a burst 1 within 5 (make it burst X within 5*X, where X is tier?); they can punish attacks made by creatures in their ZoC that do not include them as a target, and penalize such attacks as long as they have LoE to the zone. Give them the ability to hide behind allies, and they can give enemies nighmares where they're right in front of the squishy wizard, and still have to choose whether to attack the wiz and eat a SD hit (and still maybe miss) or wander around looking for the annoying Dancer.