While not perfect (this is a playtest, after all!), by and large the first two classes to debut in this packet (the Druid and Ranger) were well done. Conceptually successful with a good mechanical scaffold. I can't say the same about the Paladin. The design is solid enough, but it just doesn't hit the concept strongly enough.
That's NOT My Warden!
First we'll tackle the 800 lb gorilla (or oak, or avatar of winter, etc.) in the room. There's only the tiniest bit of resemblance to the original class here, with Nature's Wrath admittedly looking decent. The problem is outside of casting Entangle this guy isn't sticky at all. The Warden is supposed to be a black hole of nature-magic doom. I'd rather Nature's Wrath restrain targets even if you had to give up the damage.
As for his other Channel Divinity options, Lay on Hands and Turn Undead? Really? So 2/3s of his options are identical to the Cavalier's? That's not going to differentiate him at all! ALL THREE Channel Divinities need to be adaptations of stuff that the Warden could do in 4E. At least one of these should be inspired by his Daily "Form of ____" powers.
A lot of that Warden flavor is easily achievable if the designers had simply made some custom spells to support the concept. The Warden doesn't need this Cure Wounds and Bless nonsense, he needs to be able to buff himself (with a "Swift Action") in order to freeze guys in place when he hits them, or gain reach by growing tree limbs for arms, or lashing out with vines to pull fleeing enemies back toward him. Otherwise yeah, he'll just be like any other Paladin with some nature-themed parlor tricks. I think an entirely separate spell list for the different Oaths might work best. Or a pared down general Paladin list, with 2 or 3 Oath-specific spells per spell level. The Oaths simply don't play differently enough right now.
The Blackguard is in a little better shape than the Warden. At least all of his Channel Divinities reinforce his theme. Some of the same arguments still apply though, especially regarding the spell list. Why should Cure Wounds be on the general list? Isn't it a little schizophrenic for the Blackguard to have both Cure and Inflict Wounds? His Blackguard-specific options are all good choices, but it'd be nice to have 2-3 per level so he's more likely to use Blackguard spells (and still get some variety) as opposed to general Paladin spells.
Where's My Tank?
Of all the classes the Cavalier has the most justification for getting some kind of "tanking" mechanic. Selfless knight-errant, protector of the innocent and all that. Simply put, I want Divine Challenge. This guy should be saying "face me, you coward, or feel the wrath of my gods!" and have the teeth to back it up. Either damage the guy, debuff him, or both. This might work best as a Cavalier-specific spell. I mean, we're giving the Paladin spellcasting, why not use it to reinforce the archetype instead of just saying "hey, I have mini-Cleric functionality!"
With this framework the Cavalier and Warden would both actually be able to dissuade enemies from attacking their squishy allies. The Cavalier would be throwing around Divine Challenges while the Warden would be the black hole of nature magic that we've grown to love from 4E. Now, I'm not expecting at-will Divine Challenge or quite the same degree of black-hole-ness as 4E. Next is a different beast, with less emphasis on tactical combat. PCs also simply have fewer resources in general. That doesn't mean that the Next iterations of the classes can't do the same things, along the same themes, albeit in different ways, though.
Yeah, remember how I applauded the designers for not following the 3.x example and actually giving the classes their iconic abilities starting at 1st level (Druid Wild Shape, Ranger spellcasting)? Well they failed here. Honestly, the mount isn't even that mechanically significant. A lot of people really like that mount, regardless of how situational it is, because it reinforces the fact that a Paladin is a Knight with divine backing. He gets a divinely-gifted mount to ride into battle, and he's got a Druid or Ranger-like relationship with it.
That feeling still comes across even if he gets a weaker version at level 1. Something roughly comparable to a mundane horse, but with a minor Oath-specific ability. At level 8 you can get the "upgraded" version (the stat blocks as presented).
This is a contentious issue. Personally I'm of the opinion that you should be able to pick whatever alignment you choose, if you even use alignment at all. After all, each god has an alignment, and they're all over the map. If you indicate a specific god, your alignment should line up with theirs.
That said, mechanically speaking this version of the Paladin has some pretty easy-to-ignore alignment restrictions. This is exemplified best by the fact that Detect Evil has been replaced by Divine Sense. Instead of being able to detect a specific alignment (which never made much sense to me anyways; at best a person's alignment is an average of their actions and beliefs; what if an evil person is thinking good thoughts or doing good deeds when the Paladin used Detect Evil?), you can detect celestials, fiends, and undead. Cool, that actually shows that you have a connection with the divine planes, and undead are unnatural (a mockery of the divine spark of life or somesuch), so that makes sense too.
Paladins need to be tanky defender classes (except the Blackguard, he should just be really good at causing pain). A lot of this functionality should be possible by re-working the Paladin spells. They should be Paladin spells, not Cleric-lite, and each Oath should get options that strongly reinforce their theme. Channel Divinity needs to be revisited for the Warden to really let him stand out. Yeah, he's a Paladin build now and I don't necessarily mind if he'll be wearing heavy armor, using Divine Sense, etc., but the build is still conceptually more different from Cavaliers and Blackguards than those two are from each other. If you're going to use the 4E name, pay homage to the class. Finally, mounts should be a 1st level feature, and good job making alignment easily ignorable but still satisfying those who like alignment restrictions.