I'm sorta surprised that so many people are voting for advantage/disadvantage.
I mean - it's a fine mechanic and all, but it's just not that special. It's not that different from circumstantial modifiers of old in play - if anything, it's slower, because you need to roll two dice (also, I think it lacks nuance - there are definitely times when you think hmm, this huge disadvantage really shouldn't be cancelled by this minor advantage; or these two advantages should really stack). Usually it's pretty nice, but then, saying something is nice is kind of damning it with faint praise.
Let me put it this way - I'm sure the game would be affected much more drastically if it were missing bounded accuracy than if it were missing advantage/disadvantage.
So I picked two subtle ones - features I think people will undervalue, but indirectly support the game strongly.
The magic system is really good. It's good not just because it's reasonably balanced with low-magic classes, but also because it's diverse and because it's relatively small. People actually *recognize* each others spells sometimes, and that helps tie players into the world. It's not just player X casts Y from class Z (and who cares, because you can never really remember all those spells other classes have anyway), it's oh, he's casting haste, or bless or whatever, and people understand the meaning (and might even think of asking for it in an unusual circumstance, outside of combat). I really hope future publications don't greatly increase the size of the spell lists.
Which brings me to the Rate of publishing. I think it's outstanding that the rate is relatively low. Not only does that hopefully mean the content that *is* released is well-thought out, it *also* keeps the game world small enough for players to sort of understand. When there are thousands of classes, races and other significant customization options, players just don't know enough about each other and the NPCs they encounter. It's just a blur, especially for the less die-hard players out there. I'm sure there will be lots of new content, but I hope that content is sparingly doled out, excellent in quality, and of the sort that doesn't affect this player recognizability. New magic items are fine. Customizations of existing classes (as long as they're still recognizable) are fine too. Lots of new spells, feats and classes... less so. I suspect I'm in the minority - but I'd rather buy D&D 5.1 next year than splatbook 13. Give me quality over quantity any day.