An adventure map can approach a high level of detail, offering an aerial view of every bed, box, barrel and biscuit in every area. This is obviously important if you're playing 21st century D&D, with its need to regulate everything in every 5 foot square that could affect combat.
Old school module maps, though, impress with their sparse design. Apart from the keyed columns, statues, doors and stairs, and maybe some important features like pools or lava flows, the rooms of old TSR modules for the most part look empty. If it's really important to know where the chest or the painting is, you might get a letter or a symbol somewhere.
Was this just lack of artistic talent or patience? Maybe, but I also wonder if the stripped-down maps might be a security through obscurity practice. Just as the original TSR module endpage maps were printed in their memorable light blue to foil photocopying, perhaps they were also sparsely detailed to foil peeking players, or to protect against a Wikileaks-level structural failure of the DM screen. You can see where the door goes from room 26, but not what's in there; that secret is in the close-set pages of the adventure key.
I also don't think it's that important to have what's in the room drawn on the map, unless you can make it all artistic and right purty. Unless you've got the dungeon all on one page, you're going to want to open your notes up to room 26 anyway when the players break through the door. And working from the notes, you can improvise more definite locations for things if you need to, or follow simple instructions in the key like "Chest against North wall."
But what I do need to know, when I look at the map, is what's in each of the four areas that 26 connects to. If there's a loud water-driven mechanism behind the door in 27, I want to be able to describe its muffled clunkings to players who've just busted into 26. If there's a rancid smell down the 30' hall to 29, that too. And most importantly, I want to be aware if there's a monster in 25 or 28 who'll wake up and join the party if they start trying to hack through that chest with axes and sledgehammers.
Most importantly, I want to know all those things about neighboring rooms without having to write them obsessively into the key for every adjacent room.
And so ...
Color is optional, but I think helpful. Anyway, these are the kinds of thing I always forget about in the adjacent room until they're halfway done exploring it. Then I have to tell my Plato's cave spelunkers "Oh yeah, if you look through that archway it goes 20' and then there's a flickering red light and smell of roasting flesh."
Hope these will help me remember all that when they first crash in ...