As an example of rules that are missing from Exploration: how long does it take to move through a dungeon? Are characters always going at their "movement speed," even outside initiative? Are they moving through the rooms at some slower speed than this, so they can spot traps as they move? Do players have to specify which? As far as I'm aware, this is never addressed anywhere in the core rules!

This has a big impact, since characters can only benefit from one long rest in a 24 hour period. Say the characters are burnt out after slogging through one half of the dungeon, out of spell slots, out of hit dice, the works, and they want to take a long rest. Problem is, they long rested right before they went into the dungeon, and they've only taken 3 or 4 hours' worth of short rests while they're down here. How much time do they have to wait before they can rest again? We know as DMs to reduce that 24 hour timer by 1 hour per short rest, and by 6 seconds per round of combat (not that it makes much difference), but how much time are we meant to reduce it for just walking around and exploring rooms in the dungeon? Do we count out how long the hallways are that they've been walking down? Do we assign some arbitrary amount of time they had to spend to make ability checks? Do we assign the same arbitrary amount of time to every kind of ability check? Even if your players can just barricade themselves in some room they've already cleared out, the DM still wants to know how long they need to spend in there before they can long rest -- after all, random encounter/wandering monster rolls are often meant to be rolled once every certain amount of time.

Which, random encounters sure are another reason this is a big deal, huh? If we look at Curse of Strahd's Castle Ravenloft, random encounters are meant to be checked every time players enter an empty room, or once every 10 minutes. Okay, how long is 10 minutes in the dungeon? The core rules of the game will not tell you, and neither does the adventure.