I FINALLY got the playtest packet and after reading it for about 45 minutes I put my 16 year old through a session where he ran three PCs, the Dwarf Fighter, Halfling Rogue, and High Elf Wizard. I have to say this new edition will fit my casual pick up and play style well. I can also see this working for an extended campaign. 

The advantage/disadvantage system.
SPELLS are back! My player had no problem figuring out what to do with his spells and how to keep track of them. I think making so many spells "at will" helped make the Wizard more usefull even though he did go down in this particular game after slaying several goblins.
NO NEED FOR A GRID! This is one of the reasons I gave up on 4e. We don't use grids much and having to houserule all of the powers with grid based descriptions got really old. When the Wizard cast burning hands on all of those goblins, I just rolled percentile to see how many he would catch in his arc. Abstract and easy.
Healing has found a good balance IMHO between 4e's "walk it off" nerfy healing surges and more hardcore heal one HP a day type systems. The characters were attacked by town guards in the beginning of the adventure and it weakened them for the rest of the game but didn't put them out.
The old school simplicity for the monsters makes running something on the fly easier. Giving the hit dice of each monster would have been preferable over static numbers, but it was easy to extrapolate what the hit dice would be.
Combat is smoother. No more calculating your damage from so many piddly +1 or +2 bonuses from feats, abilities, powers, magic items, buffs, etc.
Easy to judge for non combat actions.  Lower DCs and lots of opposed rolls made stealth, lockpicking, tracking,etc..easier to rule on the fly and not feel like I was pulling it out of my behind.

Wizard might be a tad overpowered, but I will need more play to be sure.
Some of the monsters hit points seemed off, like the minotaur.
Some more rules for getting advantage/disadvantage in combat would be nice in the base playtest rules. It's easy to judge but a few more guidelines would be helpful.

I will have more feedback the more I run this. But I have to say you guys are off to a good start with this. I was not a big fan of 4e so this was a pleasant surprise.