My preferences would be as follow:
- generally speaking, no default setting, and only the most iconic and generic setting assumptions
- RACES: only wholly mortal, non-alignment biased, non-mechanically awkward (including level adjustment) and as much humanoid as possible races in the core: humans, elves, dwarves and halflings are must-have; half-elves, half-orcs and gnomes I prefer not but aren't a big deal; drow, orcs, hobgoblins are nice to play but I would not like them in the first PHB because of their alignment complications (but in the DMG might have a place); no undead, planetouched, genasi and other outsiders as core playable races, because of their complications on not being exactly mortals, their lives work differently; no lycanthropes or other shapechangers because of their mechanical complications; no dragonborn, warforged or other weirdos [clearly ALL these can be playable races in supplements, just not in core or at least not in PHB]
- CLASSES: only traditional classes the concept of which is common enough and large enough for plenty of character variations; I would prefer no oriental classes (e.g. Monk)
- EQUIPMENT: only middle-ages or early renaissance mundane equipment and technology
- MONSTERS: as long as the (many) iconic and most traditional monsters are in the first MM, I am not so worried here, since the DM has full direct control over what goes into her game
- RELIGIONS: I like the "placeholder deities" a lot, this is the best solution I've seen so far in D&D, I just hope we can get more than these in the PHB; I do not want a specific setting's pantheon to be default, I do not want a "best of" mix from multiple settings, I do not want a new pantheon for 5e
- ALIGNMENT: as long as it's optional, they can put the traditional 3x3 alignment; I would prefer that there be alternative also presented, but not just based on removing some combination from the 3x3, but rather something else entirely; for example, it would be nice if they presented a (simple) honor-based alignment system like in OA/Rokugan, and also they could present the M:tG 5-colour alignment system as a possibility (since they also own the M:tG brand, I guess it should be possible)
- THE PLANES: as much as I love planar adventures, avoid at all costs to assume a certain cosmology in core; spells such as "Astral Projection", "Etherealness", "Plane Shift", "Gate" etc. can all work without needing to explain the how and why exactly they work the way they do... it's magic, and all the players need to know from core is the effect of those spells, leave the details to a Manual of the Planes supplement