So what, in your opinion, makes a published campaign, module, or adventure path "good"? What published material exemplifies these attributes for you? Contrapositively, what are the elements of a "bad" published module?
It depends- I'm not entirely certain that the demands of a published campaign (I am assuming you mean "setting" here) are the same as those of a published adventure, for instance. Furthermore, what may be good for one group may be bad for another, depending on playstyle; I'll use strong stories in an adventure as an example. For a heavy-story gaming group, these might be the most awesome of adventures, while for a more sandbox-style group, they might rate very low. (Take early DragonLance modules as an example).
All that said, here's what I like, for my style. I'm speaking primarily as a dm here. I'm going to leave setting behind; even so, I apologize for rambling on... Anyway:
I want any adventure or path that I buy to be
customizable as much as I want anything. I want to feel like I can safely swap out the evil god behind the evil cult for one that actually exists in my campaign without upending the whole thing, at least if it relies on that cult and the cult is recognizably from a major setting (ain't no Bhaal in my campaign, but maybe I can shoe horn this into someone else's plans).
I want any bad guys' agendas to make sense. Likewise, with good guys' agendas. And neutral guys. In fact, I want the major npcs to have reasonable plans appropriate to their intellects. I want at least a clue as to how the bad guys respond to pc intervention; a timeline of their activities if not interfered with is great, too.
I want at least a bit of thought paid towards ecological issues- where does the tribe of goblins get its food in this dank underground lair with no exit? Is there a water source? Is there a midden, and if not, where do they poop?
I want room to expand- not like an unmapped dungeon level attached to the main locale (that's just lazy design), but lots of links to possible future adventures (a treasure map, a legend about a magic sword, the bad guy has ties to a villainous organization, etc).
I want good maps. God save us from any other published product ever that uses Dungeon Tiles for maps. Aargh! By 'good,' I mean clear, comprehensible, reasonably attractive and actually what they look like. One major problem with tiles is "This crystal thing is actually a trap door in the floor, and this pipe is really a stream." IF IT AIN'T WHAT IT IS, DRAW A GOD DAMNED NEW MAP.
I really favor a variety of encounters requiring more than just hack-and-slash, and an adventure that offers many approaches to victory without prescribing a particular one is excellent.
I do
not want encounters that make no sense and only exist to fluff the page count. Nor do I want stupid encounters that exist just to show off new mechanics. (I'm fine with new mechanics that exist just to accommodate a cool encounter, though!) Nor should an adventure remove the pcs' agency. If they have no meaningful choices to make, I can just read a book and get more satisfaction from the heroic adventures of whoever without wasting my players' time. Also, if the module ends the same way whatever the pcs do (
Pyramid of Shadows, I am looking at you), it's a waste of everyone's time.
So, a great example of good published adventure design would be
Red Hand of Doom, from 3.5. HOLY COW does this hit all the right notes! Villains with an agenda and a timeline, lots of options for the pcs, different ways the whole thing can turn out, a variety of encounters, great cartography- yeah, this one does it for me.
Bad? Try
Keep on the Shadowfell. Specifically, the encounter at the archaeological dig is pointless- there's a maguffin whose only purpose is to make the encounter have a reason to exist, that isn't even defined, never matters later and has no effect whatsoever on how it turns out. And the skill challenge with the undead guy is so utterly stupid that it hurts to think about. "Here, make a skill check with whatever you're good at!" Yeah, AWFUL.