I selected my choices based on having only played 5E at low levels, and that only as a player, not a DM.
I took the choices that were obvious to me: Adv./Disadv., Backgrounds, and Proficiency Bonus.
Adv./Disadv. is the most obvious in play. It has the appearance of being an outgrowth of 4E's "Combat Advantage," but regularized and generalized to be used in all three pillars of play. Sure! Why not make one mechanic pervade all phases of the game? That made a great deal of sense to me, and I have encountered uses for it repeatedly. To me, it's not a corner-case rule that must be memorized for specific situations; instead, it's a core mechanic that is easy to remember and easy to use, and makes the choices the PCs make have more varied consequences.
Backgrounds were introduced in 4E's PHB2; but their effects in 4E were mostly limited to a choice about one or the other of a couple of skills associated with the background: either a +2 bonus to checks involving the skill, or else inclusion of that skill in the PC's class skill list. (Some backgrounds also granted proficiency in a language.) The improvements in 5E are significant: two skill proficiencies, and tool or language proficiencies. This shifting of the burden of part of each character's choice of skill proficiencies from class to background greatly opens up the design space for diversity of characters, and I heartily approve.
I put Proficiency Bonus third because it's another of those things that went from being a niche, corner rule in the Weapons table to a much more widely-applicable attribute of each PC. Such increased generality is good.
I did not vote for:
Bounded Accuracy, because its presence is hidden in the underlying bones of the game, but does not appear in its own guise as "Bounded Accuracy" during play. It is a wonderful design principle; but none of my characters interact with it at all. Further, I have only gotten above 4th level once in 5E, and that was a playtest session wherein we deliberately started at 12th level, so I haven't yet had the experience of seeing how low-level threats remain scary as my characters slowly go up in level. This makes it hard for me to find it enthralling.
Magic Items Not Required, because I haven't ever personally seen the Christmas Tree Effect in play. I know 4E had an "intrinsic bonuses" option that removed the need for the Magic Item Escalator, so this doesn't seem too much of an innovation to me.
"Old School" feel, because I never played the old-school versions of the game.
Simplicity/Light rules, because they've lost some exactitude in the transition between 4E and 5E. Yes, simple is good; but there are still things that need to be explained more clearly. I guess that could be part of the Rulings, Not Rules philosophy; but I also guess that they will find they can reword their ideas much more clearly in future releases. I think 5E is a great step forward in the direction of Rules Light; I also think they have a distinct opportunity for further improvement.
Speed of Play, because I have been playing PbP, and "speed of play" doesn't come up in that format.
