Mearls speaks about the bad, the good, and the etc of the playtest. I am going to address some of these things out of order, mostly so I can keep all the class stuff together.
I dug the cleric quite a bit. I kept proposing a magic system in which the cleric would get “miracles” per day (instead of having to prep them), as well as weapon and armor proficiencies, and spells set by your god, and that is largely what we got. Does turn undead need to be a thing that all clerics get? Do all gods care about that sort of thing? I do not think so, so making it a spell or a function of your god makes more sense.
Why not go back to how it was handled in 4th Edition? You get Channel Divinity, and can use that on specific things, so clerics worshipping gods that hate undead can use it against them, while clerics of war gods can burn it to bolster allies in combat? On the other hand, is it even needed? It sounds like a cleric spell by another name, after all.
The fighter is, I think, the best it has ever been. While I enjoyed the warblade from Tome of Battle because it gave melee fighters diverse and useful things to do, such things were couched in supernatural or semi-magical feats (not like Feat feats, but feats like exploits). 4th Edition made a purely non-magical fighter that worked. No prestige class, no non-functional feat chain. In a similar vein, 5th Edition’s fighter is purely mundane but still has a lot of useful and flexible things to do on a round-by-round basis.
Personally I do not mind a rogue doing the whole hide, attack, etc routine. It makes sense in the narrative, with the rogue hanging back and waiting for an opportune moment to strike. I talked about this before, where at higher levels even a rogue attacking every other round can still out-damage plenty of other classes. I also think that it is a nice middle ground between 2nd Edition and 3rd and 4th Editions; instead of making it very difficult or easy (and frequent), you have to set yourself up.
Even so that type of tactic is not for everyone, so Sneak Attack becoming an option could be great for those that want to make a highly skill character that is also not good at randomly spiking damage. The idea of a smooth-talking con artist being able to distract or confuse enemies has merit, so long as it does not become a dialogue spam-fest most of the time.
While I can understand the warlock getting a nerf, the bit about the sorcerer straying too far from its identity confuses and concerns me. The only issue I took with the sorcerer was its twin soul flavor. As I said before I do not like the sorcerer having one of the sorcerer’s souls taking over when it casts lots of magic, but rather having its origin manifest (again, I will use Howl from Howl’s Moving Castle as an example of this). I really hope this is what Mearls is referring to, and not going back to 3rd Edition’s largely hands off approach to the sorcerer.
3d6 damage for a ranged attack with no restrictions is pretty heavy. Rogue’s have to work for their d6s, and even fighters pouring all of their Expertise Die cannot reliably match that damage...well, unless you got a particularly good Strength score and/or use a two-handed weapon. I can see it being dropped to 2d6 or 2d8, especially if they provide favor-fueled invocations that allow a warlock to boost it.
I...guess wizards could use more hit points? Personally I do not mind them being the iconic 4-sided wonder, if for no other reason than to give it something to do besides dagger duty. At any rate I am happy to see wizards getting more magic system types, instead of players having to settle on a magic system at the cost of concept and flavor.
Healing in 3rd Edition either demanded the use of magic or a lengthy recuperation time. This lead to somewhat silly moments where the cleric would heal the party, rest, heal some more, then rest again. In 4th Edition healing became much easier to come by (multiple classes offered healing or temp hit points), especially when out of combat (healing surges).
While healing surges made it easy to cope with time sensitive adventures, in hindsight I feel that it removed tension in the long term: in a fight as healing resources dwindled, it spiked, but once combat ended you could just top your hit points off. The only part where it started to matter was when you started to run out of healing surges.
Having recently started up a 3rd Edition Age of Worms campaign, the lack of a dedicated healer predictably makes the game more tense and dangerous. It is unfortunate that the solution is to spend lots of money on a wand of cure light wounds (or multiclass into cleric, or to roll a cleric when someone dies/becomes bored).
Ideally I would like a system where magical healing is not mandatory, and while Hit Dice (and do a point, Parry) help alleviate it somewhat (it certainly helped in both Blingdenstone sessions), my experience with 3rd Edition tells me that it will not be sufficient in the long run. Hit points are given more elaboration, in that the first half represent largely harmless scrapes and the like; divying up hit points and making it so that the first half are easily restored (or are automatically restored after combat) could help reflect this and increase survivability.
I felt that the monsters were not threatening enough because they could not really hit anything. I do not think that PC damage needs to take a dive, and I am not sure how you would go about doing this: decouple it from ability scores? Reduce weapon damage overall (make it a flat number)?
I would reintroduce 4th Edition’s proficiency bonuses. It would reduce accuracy for characters using heavy weapons like axes and hammers (which could also reduce damage), and give lots of monsters an accuracy boost. I guess it ultimately depends on how close monsters cleave to character generation rules.
Ease of DMing
If “new monsters”, opportunity attacks, and adventure creation is too much, then I suspect that person should not be a DM in the first place. The monsters are largely follow a “roll to hit, roll damage” routine with a few exceptions such as the stirge’s attach, the gelatinous cube’s slam, and the kobold trap smith’s alchemical bombs . The rules for opportunity attacks are dirt simple, and I hope that Mearls is not considering removing them (or really even making them optional).