For me (as a DM or as a player), great storylines, interesting background information, and life-like characters are an absolute must. Unexpected twists in the storyline are pretty much 100% necessary, too. Just like movies and books, If I can guess the plotline and ending of a story in the first quarter of the story, the rest is simply a really long and tedious epilogue. This boredom is magnified a thousand-fold if I have to continue to show up for games every week for 4+ hours a pop...
Having a known plot and resolution in the opening scene of an adventure is pretty much the same thing as rail-roading. Having NPCs without any depth is pretty much the same thing as the DM telling his players, "Don't worry about role-playing; he's just another 6th level fighter from whom you can buy magical swords." Additionally, having storylines and adventure concepts that are not generic enough (i.e. most Kalamar modules; they are GREAT, but they are way too Kalamar-specific for me to want to convert to my game).
Puzzles, monsters, wierd encounters, and such do not thrill me. Chances are, every store-bought adventure will have to be altered to fit my game and the campaign I am running. I might certainly keep some of these aspects if they are truly interesting, but they will certainly NOT influence my decision on whether or not I want to buy it.
Personally, I don't care if anything is Open Content or not; I don't plan on writing anything for any d20 games. Whether something is OC or not has absolutely nothing to do with buying and using a book. As for Web Enhancements, I'm not a huge fan of them. Generally (especially with WotC), Web Enhanced material is either (A) Material that was not good enough for the book in the first place...or (B) Material that is actually REALLY good but was cut from the book to make room for some other asinine monster, prestige class, or uber-kewl magic item (this just irritates me to think that publishers can find a reason to include substandard material in a $30+ book while better material has to be excluded). Maps are the exception; Map web enhancements are always good...