I like it, I like it a lot
I have played a lot of different RPGs - a lot of different systems, and have done a bit of playtesting too (Playtested WFRP 2e, and Warhammer 40'000 RP: Dark Heresy). I playtested this with one of my long time gaming friends. Someone who like me has been playing RPG's since AD&D 2e.
my initial thoughts:
1. I like HOW WOTC are going about this playtest. Don't overwhelm your playtesters with THE WHOLE DAMNED GAME IN A BADLY FORMATTED WORD DOCUMENT. There's enough to be able to play, it uses premades so you are from the start focussing on playing the game and examining the basic mechanics. So there is just enough stuff to get you thinking.
2. I think the basic mechanics are very elegant and simple. Simple enough to be adapted and improvised with - which is so so important for the success of a good rpg system in my opinion.
3. The combination of a reduced vancian system, and at will cantrips works nicely. Some of the cantrips feel a tad OP at the lower levels, but I don't think that's a huge problem as they won't scale with level - they hit about as hard as a longsword (the combat ones anyway).
4. Finesse weapons using Dex by default without needing a feat - I cannot tell you just how much I love this.
5. Heavy armour needs something more - AC is no enough - I think it needs some Damage reduction or something because otherwise it just doesn't seem to be worth using at all. Maybe some (though not so much) for Medium armour too. Make Light armour all about avoidance, and heavy armour all about soaking it, pitch medium armour somewhere in the middle.
6. the way characters are built in the basic rules here (or at least, how i can guess they are built looking at the character sheets) is great - race + class + background + theme - it works so very well. I have players in my regular group who typically take hours and hours to write characters - using this system even they would be able to make a character in 15-30 minutes.
7. This edition seems to be heading in the direction of treating its audience as though they are actually intelligent, creative people. People who do not need to be spoon-fed 4 pages of rules to explain how to trip someone over. I like that - 3e and 4e were way too defined - too much numeric certainty about what could be done when etc. 5e seems to be crediting us with the ability to play as reasonable players, to be creative and intelligent with the rules, and to cooperate with our gm's and otehr players to get the best out of the system.
We went for a walk after our initial mini session, to talk over our thoughts. As we always used to do our best thinking while walking and talking

. And while doing so came up with some interesting ways to mix & match the backgrounds and themes of the premade characters to make different feeling characters. Using slayer on the wizard for example for a more brutal wizard who overcharges their spells so that even when they fail or miss, the feedback causes some damage. Or the Slayer rogue, for an assassin type.
Also managed to come up with a theme and background to represent something i love - Runners from the video game Mirrors edge. Black market messengers adept at freerunning, and dodging the law to deliver packages and messages.
All that in a 45 minute walk. I have moved from quietly optimistic, to genuinely looking forward to seeing where the game goes and hoping it can keep this free-form cooperative storytelling feel.
WOTC you're doing a good job here, don't mess it up - add in the complexity that the 3e and 4e players want, keep it all optional so we can all play the way we want. Stick to your design goals and you will make the game you aimed at - you are definitely on the right track.