I'm going to preface this by saying that I haven't read through much of the previous D&D Next playtest material; I've skimmed it, but since I don't have an opportunity to play anytime in the near future, I didn't really go in depth with it. I downloaded the newest packet earlier today, so here are my thoughts on the latest iteration of D&D. I haven't played D&D Next, and I'm mostly comparing it to my preferred edition, 4E. For this post, I'm only talking about the How to Play section; I'll be adding another post or two on the classes, spells, and other stuff later on.
How to Play
Reading the How to Play file, a couple things are jumping out at me. One, 4E's four defenses have been done away with and the game has gone back to AC only plus saving throws. I don't know that I like that, but splitting the saves into six sections by relevant ability scores looks like a good idea. I like that the ability scores are more relevant than they were in 4E, but I'm of the opinion that they could have done away with the traditional 3-18 scale in favor of making the modifiers the actual scores.
Perception has been changed, going to a "either Wisdom or Intelligence, depending on the situation" model. It's interesting, with Wisdom being used for general noticing of things, and Intelligence being used when looking for something specific, like searching for secret doors. At first glance, I like it more than the "just Wisdom, period" way previous D&D iterations did it.
Okay, so they've gone away from 4E's move/minor/standard way of doing things and gone to action/move. I don't know if I like that, because it's looking like there's going to be less stuff that players can do per round. On the upside, a lot of stuff like drawing a weapon, digging a potion out of a backpack, and things of that nature don't take up any part of an action, rather than being discrete actions of their own as in some previous games (3.X comes to mind).
A lot of the actions you can do in combat are described as opposed ability checks, like Strength vs. Dexterity. I like it, since it gives everyone a good set of actions that they can perform outside of their class features, and they're all using the same basic mechanic.
I'm not seeing any Delay Action for combat. There's a Ready Action, but a readied action requires you to specify a circumstance for your character to act, and if that circumstance (such as "when the troll comes out of the corner") doesn't happen, it doesn't look like you can take your readied action. If it was me, I'd either have a general Delay Action where you can act anytime in the initiative order lower than your original initiative, or have both Delay and Ready Action where you get a bonus for having a specific circumstance that you choose to act on.
So far everything I've seen has been tied to an ability check in one way or another. Melee attacks are Strength checks, ranged attacks are Dexterity checks, magic attacks go off the relevant ability score (Wisdom/Intelligence from what I've seen). It's not bad, actually. I like that everything is mechanically extremely similar; it's taking 3.X's philosophy of the core mechanic being a d20 roll and taking it one step further.
One thing I am immediately not a fan of: the return of bludgeoning/piercing/slashing damage types. I'm cool with stuff like necrotic or psychic damage, as they're supernaturally different, but for some reason I've never been a big fan of the weapon damage types. Casters will (hopefully) have access to different spells to cover stuff like fire or cold damage, but this means that, just in case, weapon-based characters will want to carry around separate weapons (mace for fighting skeletons, sword for other stuff, etc.), which is going to be a problem if a character only has one magic weapon that doesn't work effectively for a particular combat.
They've done away with the bloodied condition, something I don't necessarily agree with. Now, death is your level plus your Constitution as a negative, so if you're level 5 with a 14 Constitution, you die at -19 hit points. I think it was easier to use bloodied as a measure of your health and as a death threshold (negative bloodied value), but this should also work. I just prefer the bloodied condition.
Spell components and armor casting are back, something I have never been a fan of. Granted, it makes sense to balance casters against martial characters when you have stuff like sunder or disarm in the game, but I wasn't a particular fan of those actions either.
Concentrating on spells is back, though it doesn't look like you can interrupt a spell like you could in 3.X. Instead, it's for prolonging sustained effect, something I don't have a problem with. So, that's an upside.