The DM makes the module fun. The DM tailors the adventure to fit the players styles, tactical aptitude, and party composition. The DM is also responsible for giving the PC's fair warning through either divination and/or sagely personages.
I have turned many a module that people have said sucks into adventures that my groups have talked about for months afterwords. With a smile on their face and excitement in their voice. The "money" for a DM's work.
They often, to my surprise, remember many NPC's, and not just the "bad guys". They have remembered quirky blacksmiths adn apothecarians, the brooding armor smith, the grouchy old jeweler, and many others. I had no clue as to how much my players liked such characters until they revisit the town they were met in and then go look them up. I once used it as a excellent adventure hook. When the party finally found the gem cutter I initially had him dead, but when I saw how upset the expressions on my players faces became he suddenly had a barely perceptable pulse.