Well, I’m probably not the best person to answer either, seeing as I’ve been here two-and-a-half years and have never started a thread that reached a second page. (Though my “current thread,” What will your biography be called?, is now at 45 replies.)
However, I think a lot of the really really long threads here fall into well-defined categories:
- Simple polls (“What’s your favourite colour?” and the like)
- The “traditional threads” that everybody knows about (Esprix’s “Ask the Gay Guy!,” Crunchy Frog’s “Welcome Wagon,” etc.)
- Sex threads. Unfailingly popular.
- GD threads, especially if one or more of the posters is a jerk.
So, I guess the “perfect SD question” would be something like “Ask the Gay Guy what his favourite colour is when he’s having sex!” But only if the OP is a jerk.