Wow. Finally digested the rules, the DM guidelines, the characters, and skimmed the monsters.
My VERY first impression of the rules: While I liked the dis/advantage thing A LOT, most of the rest was making me go, "Okay, so it's a simplified form of 3e. Groovy, I guess, but a little meh." Then I read the spells and the character sheets and my brain nearly exploded with awesome.
The spells are thoroughly old-school. ME LIKE! But there's more...
* Turn Undead is a spell! Just like it should've always been! It's just that the Channel Divinity class feature (a 4e-ism, I think? but one I like) can let you cast it a few times for free. It's just plain better than the 3e version, too. (I forget off the top of my head how the older versions worked.)
* Am I missing something? Or are clerics now freakin' spontaneous casters?! I can't believe nobody's mentioned this yet, and it's AWESOME! I've always thought they should be.
* The backgrounds and themes are quite evocative, better than I had thought. The Slayer theme is appropriately terrifying. The Ambusher feat the rogue has seems completely redundant, as someone already mentioned. 'Forbidden Lore' just oozes gooey fun.
* Some halfling clans are hobbitty, others are kendery, and yet others are Gypsy street gangs. A clever way to get around all the takes on the race in the past.
* The cleric of Moradin is 3/4 of the way to being a paladin already! Do we really need the class? REALLY? In general, the idea that clerics aren't all priests opens up all sorts of worldbuilding vistas.
* I just about cheered when I saw the 'Improvise' action.

And the GM advice is refreshingly empowering.
* It's a little odd to me that Wis saves are used for illusions, I would have thought Int. Int saves don't seem like they'll see much use. Wis saves seem overused generally, why not a Cha save for charm?
Overall, very happy. I can't wait to try this out!