Qualities of a Great Old School Module

I saw this comment by Guy Fullerton in speaking of the module The Shattered Circle (TSR 11325, from 1998) here and thought it really worthy of note:

"It has a lot of elements that remind me of ~1980 era releases: Ease of dropping this almost anywhere in a campaign, a fairly open dungeon layout, some good traps, player (not character) puzzles, enigmas, potentially unbalanced encounters, places that strongly promote DM improvisation, and plenty of opportunities for the party to make choices between fighting, communication, or something else entirely.

It has a backstory, and although it supports a rather standard altruistic way for the PCs to interact with that backstory, it by no means forces the party to do the altruistic thing."

I would love to explore each of those points a little.

1) Ease of Dropping Into a Campaign

This seems related to my post about Detail being dominant. It seems a hard line to walk, but the module should provide interesting details without pinning me down. Things like very specific gods, historical events, and fleshed out cities and cultures would probably be no-nos. I'll decide if my world has viking-mole-dwarves or not, thank you very much.

2) Open Dungeon Layout

I'm guessing this means the dungeon isn't linear, there are multiple possible paths. Sounds good to me, and relatively easy to take into consideration while designing, except if the location is a tower or something similar.

3) Good Traps

I don't disagree, but I'm not sure what good traps would be. Traps that can be seen by players, strategized against-- not instant, silent killers? Or just interesting, as in not another pit?

4) Player Puzzles

I love these myself, well making them anyway, and realize how difficult it can be to make them challenging without frustrating. If a module had one or two puzzles that managed this successfully, it seems that would be a plus indeed.

5) Enigmas

I like this too. One problem is that players tend to think every detail is significant, so if you mention a lever, it must be important. But why shouldn't there be weird atmospheric effects, or non-threatening oddities every now and then to please the explorers among us?

6) Potentially Unbalanced Encounters

This goes hand in hand with 2). Without this openness is only avoiding the feeling of being railroaded. With this possibility, openness of the dungeon becomes strategically imperative. But make sure to give the players clues to approaching encounter dangers, or the only choice they'll have is whether to run or fight.

7) Promotes DM improvisation

I strongly like this as a suggested feature, but in my inexperience am not sure how I would provide this to DMs in a product of my own design. Present NPC motivations and let encounters work themselves out? Provide clues, but only brief ones, as to what could happen if players take certain actions?

8) Opportunities for Choices Between Combat/Communication/Other

This seems related to 2) and 6) but still worthy of a goal to strive for. Seems like this would weigh against modules with only dumb, hungry jellies or rabidly fanatic cultists. Try for conflicted, intelligent humanoids of some type.

Any other points that Guy hasn't touched upon that you think a great module should have? I'll add my own two cents with:

9) Re-visitability

It should be a location that would be interesting to revisit later. Perhaps there are magical fountains or pools that are permanent features. This doesn't mean a module couldn't be about a collapsing cavern or erupting volcano, but if the DM has to do the work of importing it into the campaign world and learn the details enough to run players through, it would seem like a nice touch to make it a landmark of sorts, a place to come back to.

And . . . I can't think of another to make it a nice round ten. Let me know if you have any more or disagree with any of the nine here. Thanks to Guy Fullerton for the great quote.

Update: Maybe a tenth would be

A Reason to Visit, but Ignorable

The players can be altruistic and helpful, but they can just be looking for gold if they choose. But it requires a certain amount of narrative to provide a problem the players might help with. And it takes some skill to not, then, require PCs to follow some expected course of action.