I have been running some adventures on RtA lately and, as such, I've had a few thoughts about how to make a good, exciting adventure for players. As I start to cast my hoary eye finally on Begma and begin to work up its overarching plot, I will likely be running more adventures in the future.
These are just my random notes in absolutely no particular order. They aren't comprehensive. They aren't endorsed by Staff or the Powers that Be. Perhaps, though, my notes are helpful to someone else out there who wants to do Cool Things.
1. A great primer is already available for anyone who wants to start running adventures. Start here: Creating Adventures.
2. 3-5 players is the ideal group size. Any smaller and it is difficult to keep banter going. Any larger and it is difficult to keep the entire group moving. A banter-movement balance is really desirable.
3. The PCs in the adventure (sans the GM's PC) are the most important people in the story. NPCs should provide monologues, hints, hooks, and help, but they should never be in the spotlight.
4. When planning for time, assume a good combat will last an hour of real time. This is a rule of thumb: it may last longer for more complicated fights or shorter for easily dispatched opponents. Combats seem to hit that hour mark.
5. A good simple structure for an adventure is intro -> encounter -> encounter -> climax. Encounters can be combat or some other sort of interesting interaction (like Jaguar Women of Ciabola) or fleeing from something horrible. An "encounter" does not necessarily mean sticking your sword into someone else. It can mean a political defeat, achieving a goal, navigating a dangerous situation or discovering a set of (hopefully horrible) facts.
6. The climax of the adventure does not always have to be a combat. It can be learning a hidden secret, finding a hidden location, or achieving some sort of great political deed. But it should be satisfying and conclusive and hit an emotional high point.
7. Puzzles should be structured so they have opening/presentation but no definite answer. This is important: there is no predetermined winning condition and no expectations of brilliance. Let the players come up with the answer as a group and roll with their solution, even if the solution presents more problems. If the solution to the oncoming steamwork mecha army is trebuchets loaded with tonnes of cherry tart, then the PCs launch the cherry tart and it arcs through the air with glistening red drippy glory.
8. Keep your challenge ratings within reason of your players.
9. Keeping things moving is important. If the PCs seem stuck or lost, it is time to simply move them on to the next bit of adventure even if it means a little deus ex machina. NPCs should point the way, the PCs should get maps, the trail should become obvious, etc.
10. Try to spread information around to players. One person hears something, the next person sees something, another person knows a fact, etc. Use their gifts and who they are to the best advantage to give them information. People with danger sense or high lore knowledge are exceptionally useful for receiving instant bits of information for free. Players paid for this information and its on their sheet; a GM should use it to give them the advantages they paid for.
11. I have a deep fondness for foley and lighting. Meaning: I like sound effects and light effects. I tend to pose them in the scene to convey a sense of area and world. Black steamworks send up poofs of black choking smoke. Flying sawblades make szzzzt noises. I have no idea if this is annoying or interesting but I enjoy it.
12. On a personal note, I vastly prefer to run adventures with my GM NPC object than my PC object. If possible, I prefer to leave my PC at home but sometimes it is unavoidable. When I have him, I use him as a GM tool.
13. And whenever you are stuck and you can't think of anything else, you can always, always, always, always send in ninjas.
None of this is set in stone. A really awesome scene can happen with 1 player and 1 GM. Long running plots may mean a series of encounters to build up to a final climax. Nothing has to fit this narrativistic structure at all.
YMMV and all that.