The Title, Cover Art, and "blurb on the back" all need to work together to attract buyers. The module needs to be about something unique; hopefully, not something I've played or run before. WHY are the PCs summoned to investigate the Wizrd's tower?
All of that requires the adventure to be about something, and stick to a particular theme. Then, if the theme interests me, I might pick it up.
No railroading also means that there should be more ways out of a situation than just "use magic... or die". What about the skillfull Rogues, or lesser skill-monkeys? Encounters which hinge upon the PCs having one spell are as bad as adventures which are "over" because the one NPC dies... Avoid both.
Adventures should be adaptable to a multitude of styles of play. If the party is not fighter-heavy, or has no Rogue, or whatever, there should be ways of avoiding or overcoming the obstacles and challenges without them. This also relates to NPCs.
NPCs should have motives for doing what they do, and living where they are. PCs should be able to find out these motives. Instead of fighting the kobolds with a party that is fighter-weak, perhaps the Bard could learn why the kobolds inhabit the old mine, and work out a deal to get past them, without bloodshed. Thus, Hack'n'Slashers and Role-Players could both enjoy the module.
Reward good ideas. I have yet to see a module where doing research into the history of So-and-so's tomb (or whatever) did any good... If the wise King Herremann was buried in his royal crypt, the builders probably had a floorplan, too. Why can't a PC make use of their Knowledge (History) to know stuff about the place? (And even so, things can have changed, in the 500 years, since, and the kobolds may now be running the place!) Another example would be when the PCs see a statue of the King in the tomb. Someone with high Knowledge (History or Local) could recognize him from his image on a coin in Uncle Sal's collection, or somesuch... but I have yet to see this done, in D&D...
Don't negate character abilities: Hordes of Constructs & Undead for the Rogue, trackless wastes for the Ranger, areas of anti-magic, or anti-teleport zones for the Wizards, "Psionics don't work, here" for the Psis and PsiWarriors, etc. It is a CHEAP tactic. Instead, allow the abilities to work as normal, perhaps with a -2 Circumstance Penalty. Occassional "nerfings" are okay, but area-wide, adventure-long versions are outre'! Find a better explanation... the Wiz can't usually teleport EVERYONE out, anyway.
Once in a while, it is okay to give the PCs insoluable problems, but usually, it is better to give them multiple ways to solve them, and then let them come up with their own version of a solution... Faced with an uncrossable gorge, the PCs could come up with the Wiz casting Tenser's Floating Disc, which the rest stand upon, whereupon he casts Jump, or they could find some ladders in the dungeon, which they tie together with rope, or whatever. Again, there should be multiple ways out, so that parties who miss one can find another, regardless of whether or not they have the proper spells prepared.
I agree with much of what was said preceeding this, about NPCs and plots having logical motivations, etc. NPCs should all have reasons for what they do, besides "the Wizard is mad". Even if he is, WHY is he releasing Owlbears on the Kingdom? What does he hope to gain? Double-ditto for the "mad gods" blamed for so much... Get rid of all of them, and get better explanations.