What would make the game more playable "out of the box"?
I think there are a number of things that could make the game more playable "out of the box" -- some have been touched on; some haven't.
Umbran mentioned "a drastic enough simplification of the game to make it restrictive". That sounds awful, phrased like that, but a simpler set of core rules would certainly make more sense to new players -- and it's not as if they can't buy supplements later.
Agback made the excellent point that "in D&D so much depends on knowing details about features of the D&D game world" and that a more "realistic" world is easier to understand and gamemaster.
Of course, saying "realistic" in a D&D conversation summons all kinds of trouble, but he has a point. Once characters can ignore common soldiers, fly, teleport, read minds, divine truth, etc., it becomes much, much harder to plan an adventure. You'll notice that low-level adventure modules read more like common fantasy stories. High-level adventures read like pure D&D.
So we simplify the game a bit and we reserve the reality-bending magic for higher levels. That helps a bit. What else can we do? For all its many pages of advice, I don't think the DM's Guide lays out the typical adventure formula quite clearly enough. I'd like to see something as crystal clear as
The Lester Dent Pulp Paper Master Fiction Plot. (Lester Dent wrote the Doc Savage stories.) Once you know what you're doing, you can drop the formula. A snippet:
Here's how it starts:
- A DIFFERENT MURDER METHOD FOR VILLAIN TO USE
- A DIFFERENT THING FOR VILLAIN TO BE SEEKING
- A DIFFERENT LOCALE
- A MENACE WHICH IS TO HANG LIKE A CLOUD OVER HERO
Everyone could also use certain locations mapped out, preferably full-size, as battle maps: a tavern, a tower, a keep, a monster den, etc.
Everyone could also use pregenerated stats for certain common monsters/enemies -- not just the 1st-level Goblin Warriors, but their 3rd-level Captain, the Wolf-Riders, etc.
What would you put in a Basic Set?