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In my mind all the content stuff described so far helps define a GOOD adventure, whether or not it is published.
If we're talking about GREAT *published* adventures, then in addition to meeting the requirements for a GOOD adventure, I think usability/adaptability and convenience are the defining features. D&D varies a lot from table to table, and an adventure which can adapt to different play styles is on its way to being great. Also, if I'm turning to a published adventure rather than my own it's probably to save time (unless it's a classic D&D adventure, in which case it's for nostalgia); so I need an outline or flowchart of events, an index or table of contents, call-outs pointing where the PCs might go into uncharted territory and how to handle it, some guidelines on scaling the adventure for party size/level/composition, clear maps, useful player handouts, etc.
If we're talking about GREAT *published* adventures, then in addition to meeting the requirements for a GOOD adventure, I think usability/adaptability and convenience are the defining features. D&D varies a lot from table to table, and an adventure which can adapt to different play styles is on its way to being great. Also, if I'm turning to a published adventure rather than my own it's probably to save time (unless it's a classic D&D adventure, in which case it's for nostalgia); so I need an outline or flowchart of events, an index or table of contents, call-outs pointing where the PCs might go into uncharted territory and how to handle it, some guidelines on scaling the adventure for party size/level/composition, clear maps, useful player handouts, etc.