Good points. I also agree with the title thing: I'm much more likely to click on a thread with a specific title, as opposed to a generic, "Who was rude here?" title. It's also easier to remember which thread it was later--if a thread turns out to be a good one and you want to tell people about it later, it helps if it has a good title!
I do think derailments of most kinds can make a thread "less good." That's another way that the thread could avoid answering the OP's question (along with criticizing something else in the story or suggesting alternatives that the OP has said they don't want), and I think the purpose of most threads here ought to be to answer the OP's question. (Of course, there are exceptions, especially in folders like Coffee Break, where it's more about people getting to know each other, and that's fine.)
It can be difficult sometimes to know which details are important to put in, and which can be left out. Though, I think sometimes the "non-essential" details actually help the posters understand the OP's real problem and give advice on it, even if the OP doesn't see that as the real problem at first. For example, the OP might start off by asking if it's rude to bring her own food to her MIL's BBQ, and gradually it comes out that MIL is very controlling and petty and point-blank refuses to serve anything the OP can eat, so the OP's problem is a WHOLE lot bigger than just one BBQ, and thus people can give advice about the REAL situation. It seems like those kinds of threads might potentially be really, really helpful to the OP, if they're given a new perspective on their lives and encouragement to change things.