Now that I have a session of 5E under my belt I have a few comments on its design and functionality.
Advantage and Disadvantage are useful enough. They don't really solve the whole problem of needing to track and add bonuses to things as any number of spells still add or subtract digits from die rolls but they do solve most of it. I suppose that's enough.
The swinginess of the system isn't as noticeable at low level, where we expect play to be very swingy and dice dependent.
The spellcasting systems are too complicated. I need to redesign part of the character sheet to make it clear to the people with Cantrips that those are powers with infinite uses. Right now the fact that there are casting rules for one set of spells and other rules for a different set is messing with the players. Things are going to get worse when the Sorcerer levels up and there's a third set of rules for her spells distinct to the sorcerer class. This might not be a problem to people coming in fresh, but its enough not like 3E and Pathfinder that it grates and slows play.
The Kits, er Backgrounds, need to be clearer in how experienced the PC is. As it stands its possible for your first level PC to be a local hero, a respected officer or an established member of a spy network - all of which imply a certain level of life experience - or an untested acolyte or street urchin - which implies a very different one. It's a minor nit, but since the experience tables treat levels 1-4 as the Design In Play, your kids from the backwater village become legitimate heroes levels, it became a noticeable one. The actual abilities conveyed by the backgrounds don't have to change, just the text for them.
I'm not sure we need the subraces; they add another level of complexity to what could be rapid character creation (Race + Class + Background; select 2 skills from list, done) and I'm not sure how much they add other than more need to memorize crunchy bits.
Likewise I don't think I'll be adding Feats to my game. I found them critical in 3E, but now I feel like they're in the way. Has my gaming style changed in the last 14 years? Sure. Now I feel like the people who saw Non-Weapon Proficiencies as being optional in 2E and treated them as optional, as opposed to what I did back in 89 which was treat them as critical.
I get the real sense that 5E is trying to reduce the advantages of system mastery, but didn't want to go all the way on that, so Feats are still there but 'optional' and classes have lots of little options. I want to see system mastery go the way of the dodo in this edition, however. Hence some of my decisions.
I also told the players that they had to go with either the standard spread or roll at the table and take what they got. I then had to wrestle one of the adult players to the ground because he immediately insisted his character concept required a point build so he could spread things around and not have the '8' that comes with the stat spread.
I do have to admit that when Nick rolled up his elf paladin and the dice hosed him (he has more penalties due to stats than plusses) he took it with impressive grace. Jason admitted later he'd probably have groused more than Nick did. Nick then took our advice and borrowed things from the Dragon Magazine article I wrote years ago about how you can use bad stats to make your character impressive by having his background - being the sole survivor of a red dragon attack that killed his village - be writ in scars across his body to explain clearly why he has a below average Dex and Con. Yes, he has below average stats, but since he should be DEAD, and you would be DEAD if you'd been there, his stats are pretty damn awesome. It makes for a visually and socially impressive character, and fortunately 5E is not stingy with increases to stats over play.
I'm looking forward to next session.