1. Names are a big deal for me to. I hate modern sounding names, silly names (unless it's an unashamedly silly module), unpronounceable names, and I'm growing to really detest flowery elven names like Tallwillow Treestrider and such. Real world names are ideal as long as they don't sound too modern. Babs, Woody, Jim, Bubba, Vicky, Nicky, and Mike as the named NPCs would ruin the fantasy feel of a module. If those names were Heraclitus, Cicero, Julius, Perpetua, Sextus, Quintus, Hestia, and Penelope, it would work well (as long as it was supposed to have a Greco-Roman feel). If the names were Hallbjorn, Hjallti, Skeggi, William, Ungit, Siegerda, Brynhild, Gunnar, Grettir, and Njal, it'll work fine. If the names are Arthur, Percival, Vivien, Gareth, Lot, Agravaine, Morgana, Pellenor, and Gaheris, it'll work fine too. Tossing a few more modern sounding names like Martin, Peter, Michael, Elizabeth, Mary, Joseph, John, etc in among those names works fine too though if all of the names were in contemporary usage, it wouldn't seem right.
2. Overuse or inappropriate use of templates. If the first encounter has a shadow halfing, the second encounter has a fire-elemental dragon, the third encounter is feral lizardmen, the fourth encounter is a half-golem troglodyte, and the last encounter includes a Yuan Ti and a half dragon, you've used too darn many templates. Just give me some plain monsters every now and then or make the template a theme of the dungeon. Similarly, if you start statting up half-troll worgs and toss them into a module without a mad wizard grafting troll flesh onto worgs, you're misusing templates. Players shouldn't start making jokes about the troll fens where the trolls wear velcro gloves and the worgs are scared.
3. Modules whose plots don't make sense. So, you went to great expense and effort to kidnap us and blackmail us into assaulting your enemies for you? Why not just hire mercenaries who might not want to stab you in the back for more money? For that matter, what is it that we can do that you (who were able to kidnap us successfully) can't?
4. Modules that expect highly unrealistic player actions--or at least that don't anticipate predictable player reactions.
So, you'll kill the captain's village if we don't help your evil organization?
Yes, our monsters are on the way as we speak.
And what's to guarantee that you won't kill them anyway?
Our word.
And what's that worth? Forget that, I know what your word is worth; what's to stop you from threatening them again when we finish this?
nothing
The we don't buy your blackmail; eat flamestrike... Mr wizard, can you teleport us to the captain's village; surely he can give you a good description or you can read his mind to get one.
Well, he probably should be able to do that but the module assumes you'll give in to the bad guys so the adventure is over. Rocks fall and everyone dies.
5. Moral dilemmas that aren't.
So, do you support the obviously deranged heretic burning crowd that magically poses no threat to you and free the innocent, virginal victims of racist, bigoted oppression, or do you join them? Yeah right. If you want a moral dilemma, make it a real dilemma. Either the crowd will turn on you if you don't go along with them or the people are really guilty or something has to be there in order to make the choice either a choice between what's convenient and safe vs. what's right or either a choice between the lesser of two evils or between two goods. Otherwise, it's not a dilemma.
6. NPCs that don't have the skills to do what they're supposed to do. A successful merchant with no ranks in sense motive. A skilled and feared fighter who singlehandedly defeated an orc warband... and is second level with really weak stats.
7. NPCs with godlike stats for everyone the PCs meet. I don't mind the odd character with better stats than the PCs. I don't mind if most adventurer types and successful self-made men have stats similar to the PCs. But if every city guard has an 18 strength, a 16 con, and a +1 halberd and +1 breastplate when the PCs are struggling to find masterwork equipment and are left with no mental stats if they go for an 18 and a 16 (point buy), it gets rather frustrating.
8. Bad guys who act evil but aren't.
So, my legionaire of doom, who guards the slave pens and kicks the slaves who step out of line and protects my sacrificial chamber where I make the blood golems, are you evil?
no.
Why not?
If I were evil, I'd be vulnerable to Holy Smite and Smite Evil and Magic Circle Against Evil would protect against my attacks. PCs expect the servants of the evil empire to be evil so I'm going to be neutral--hey I'm just doing my job torturing innocents and beating the slaves. Even when I have fun doing that, I'm just doing what's socially expected in my circle of peers. So I'm neutral not evil.
Yeah right.
9. Bad guys who are evil because they want to be evil. Give me NPCs willing to do evil to get power, NPCs willing to do evil for love, sex, spite, vengeance, pleasure, whatever. But let's not pretend that bad guys are selflessly devoted to the pursuit of evil. They should be selfishly devoted to things that lead them into evil instead.
10. NPCs getting all cloak and dagger about stuff that's perfectly obvious to anyone who cares to look. "Swear unconditionally, that you won't tell anyone that I'm buying grain in town." As if anyone who wants to know can't just watch where the crates are coming from and figure out you're buying grain. Etc.
11. NPCs who are doing stuff they shouldn't be able to do--like unobtrusively sneaking through a crowd of workers WITH FIRE SHIELD ACTIVE. Not so unobtrusive when you're IMMOLATED IN FLAME are you? Or did the author forget to mention all the common laborers are fire elementals rather than humans like one might otherwise think.