I was flipping through my old friend, the 1981 "Moldvay" Basic book, when I happened to stop at page 19, "Part 4: The Adventure." This is a fascinating chapter to me. There are so many little tidbits that encapsulate much of what I consider "old-school" play.
One of the first sections is about party size, containing the passage:
"The best size for an adventuring party is 6-8 characters, enough to handle the challenges which will be faced, but not too many to become disorganized or to ruin the chances to surprise the monsters."
This is large compared to most present-day groups, but it's also interesting because it implies the need for numbers in a dungeon (a concept reinforced later with the mention of hiring retainers). The other interesting implication in that sentence is that of stealth. Adventurers should be trying to sneak up on things sometimes, not necessarily barge right in.
A dungeon should be a scary place, with darkness, monsters, and traps threatening to finish the characters off at any moment. Adventurers that want to prevail need to bring enough bodies to hold their own, as well as trying to gain any advantage they can while in the deep places of the world.