I've seen a couple of questions which actually were multiple questions at once. It's not only hard to answer if you don't know the answers to all of them, but also difficult for the questioner to accept an answer, if only multiple answers cover all of his questions.
I suggest to extend the pre-populated title text when opening a new question with something like:
I think there are genuine cases where you have an issue that consists of more than one question where you have a sequence of things to do and you need help along the way.
One issue per thread but as many embedded sub-questions as makes sense.
I can't think of a good hypothetical question today but I have a real example on StackOverflow: How best to convince people to upgrade IE? This is an umbrella question with several little related questions. Some people answered the main question, some people answered the sub-questions. The end result was a large block of answers that collectively impart a lot of information.
I think my sub-questions there don't derail the main question at all. They all help to concrete the scope of the issue and they also help people answer in directions that I wanted.
Picking one best answer wasn't tough. There was one answer that proved the most useful to me but lots of answers that I voted up. That's kind of the point of voting. It makes sure that all the helpful answers get to the top.
There is of course a limit to all this and that pulls me back to one issue. As soon as a sub-question becomes something else completely, there's no sense in it being part of the first thread.
It's not black, moderators have to keep on top of it... But I think it's a lot healthier than forcing people (or making them feel forced) to split up perfectly related questions when they're just trying to resolve a single problem.
