I'll start with the caveat, my words will be harsh, but I understand they are in the early stages of development, and many things can change.
My first impression is, we already have this game on our shelves (or in my case, a cardboard box in the crawl space). Its pages are a bit yellower from age, and I might have some difficulty finding my bundle of house rules, but it is the same game. When I played this "next" iteration, it felt like I was playing AD&D with a bunch of house rules, and since those house rules are coming from the house of WoTC, we're just calling them rules. I don't really see the point.
I love many aspects of 4e. I think it was a step or three in the right direction. Maybe they lost a few flavor aspects, high level play got a bit out of hand, character presentation is poor, and mapless play lost its appeal, but those are things that could be addressed in a new edition. I've run a number of 4e session where there was combat, but I used a large scale map rather loosely, and it works fine. I don't feel a map is absolutely necessary to make 4e work. I've experimented with better representation of character information, I've taken precautions in my home games so some of the high level play options don't get out of hand. I like the "DM education" concepts they are preaching for the next edition, reigning in attack/defense bonuses, and a few other aspects. But those things could also be done on the 4e framework.
I would have much rather seen them go forward, rather than this whole zen approach of starting from basic D&D, and building up again, to rediscover the game of Dungeons and Dragons. It seems like a whole lot of work to give us something we already have. Yes most of our rules were hand written and in binders, but they were exactly how we wanted them to be. And we've moved on (well... admittedly, some of us haven't).
I played a dwarf cleric in the play test (i think I'm allowed to say that), and it felt very lackluster (after having played numerous 4e characters from level 1). My healing was pointless. I found only one combat useful spell among the 6 or 7 I had access to. Best thing I could do was swing my hammer or throw an axe, neither of which I did particularly well. I didn't feel like a dwarf, I didn't feel like a cleric, I didn't feel like an adventurer who could climb, jump, come up with solutions to puzzles, or invoke the name of his god in battle for tangible benefits. It was very bland, and very uninspiring. I had an AC, I had hit points, I could swing a weapon, and I could do damage. I guess we're calling that D&D these days. We did some mindless hack and slash, and though it was challenging at times, I quickly got bored with it. There was no story in the adventure, which was quite disappointing. A sandbox is not really appropriate for a play test. When I tried to do interesting things in combat, the options were not available to me. Aside from some interaction with terrain, it didn't look like there was any flavor to anything we could do.
It also felt a little bit like a board game, where you don't really care if your character dies, you'll just get another one from the box, and keep playing. I had a sense of the character being disposable, not a very good feeling for a role playing game.
And all the elements I hated from earlier editions seemed to crop into this edition. I don't care to collect weapons and armor from enemies we've dropped, just so we can sell them at half price and collect a few silver coins to buy ammunition. I don't care to have to take the rest of the day off after 5 minutes of adventuring. I guess I just no longer like "old skool" D&D. I felt the same way about 8 months ago when I tried Pathfinder. I've been spoiled by 4th edition, the edition of heroic games. I just want to go forward from here, not backwards. In 4th edition I feel great freedom in the DM chair to create adventures, to create a campaign, exactly the way it is in my vision.
They could easily achieve their "simple character" goal they seem to have with this edition, using any other edition. The things they are talking about for the simple characters is just a glorified pregenerated character. You can swap out components for customization. So, take a pregen character, and start retraining feats, powers, stats, etc, and voila, you have a "simple" character, and a "custom" character.
I will wait to see more, to see how the next layer of development will work out, and I will look through the open play test documents when they are available, but I am not optimistic. When 4e was coming out, I was hyped about everything, scoured ENWorld for every new tidbit of information, and ran demos the moment they were available. For next edition, I'm not sure I even want to be involved in play test, or give any feedback, because it would not be constructive. I would just sound like a hater and be ignored.
In conclusion, before DDXP, I was mildly intrigued but skeptical, after DDXP, I'm pretty disappointed with a very tiny spark of hope that lots will change when that next layer of granularity is added to the system. I want to like the next edition, because they are adding some good ideas in. But they are taking out way more than they are adding for my taste.