In approximate order, these are the things I dislike most about purchased adventures:
1. Big rules errors. I'm not talking small mistakes like missing a few bonus skill points on an NPC. I'm talking blatant errors that even a casual player can quickly identify as being wrong.
2. Not playtested. Sometimes, I find certain encounters or areas have not been properly playtested, and it can lead to major problems during or after the encounter.
3. Unbalanced for the intended party EL. Sometimes hard to differentiate from #2, but I'll classify it differently. Some authors just seem to like punishing parties with over-the-top encounters. Gee, you're only 1st level? Well, here's an EL7 monster coming at you.
4. Railroading. Nearly always a bad way to steer the plot, though in the right situation it might be tolerable...barely.
5. Monty Haul treasure. If authors follow the guidelines for treasure per encounter and NPC, things are manageable. But there seems to be a temptation to put super-duper items in as rewards which can lead to balance problems later.
6. Poorly drawn/keyed maps. I understand that revisions occur, but I like to have the maps line up with the entries in the module when ever they are referenced.
7. Highly intelligent creatures w/ poor defenses or strategy. It's ok for stupid monsters to have poor tactics or strategy, but highly intelligent ones should come up with elaborate traps, tricks or other tactics to defend themselves.
8. Too highly specialized of a setting. Suppose the adventure absolutely must occur in a huge, ancient coastal city, built on the side of the tallest mountain in the world which is actually a dormant volcano, and just happens to also be at the very intersection of 4 major warring countries consisting of humans, orcs, lizardmen and fey. Just where in the heck can I place that? I may buy it to run as a one-shot, but that's likely my only option.
9. Forgetting to list info. Editting errors happen, but sometimes key information regarding a room, treasure or the occupants is left out, causing a DM to scramble.
10. Making assumptions about party composition and abilities. Not every party consists of fighter,cleric,wizard and rogue. This is bad if a key plot item requires a certain ability the party may not have at all. For example, the only way to escape the final sure-fire-party-killing-death-trap is by using the high-level arcane scroll found in a chest. What if party has no wiz/sor and no one has any ranks in use magic device?
