As this seems to have turned from a discussion about a specific type of question, into the defense of a single question. I think my comments on the question describe why the question was deemed off topic.
If you're looking for a prefabricated solution; as it sounds like you are ("Does anyone know of any sliding window actuators?"), then this question is off topic. If you're trying to design and build your own device, this question is too broad. We are here to help with specific home improvement problems, we are not a crowd sourced research and development department. – Tester101♦ Sep 5 at 15:39
The question was originally asked as a "shop for me" type question, where the OP seemed to be looking for an off the shelf solution. The question was then edited and became more broad and open ended, asking the community to design a complex system for the OP.
See my comment above. The original wording sounded like a "shop for me" type question. However, the new wording pushes the question into the "too broad" realm. There are a thousand and one ways to mechanically open and close the window, see this answer for a few ideas. Just because the question doesn't fit on any other stack, does not mean it's on topic here. – Tester101♦ Sep 8 at 11:53
The OP then asked for me to clarify why I felt the question was too broad, and if I thought the question was detrimental to the site.
I don't believe the other question is good either, which is why I provided the answer I did. Your question is too broad, because you are asking us to solve a complex problem with no guidelines whatsoever. You're asking us to design a potentially patentable product for you. As far as this question being detrimental to the site... possibly. Folks tend to use similar questions as precedent, when arguing that their question is on topic (as you have done). So leaving questions like this open can indeed be detrimental to the site. – Tester101♦ Sep 9 at 12:23
That comment gets to the point of why I feel this question is off topic.
If the OP would have asked something along the lines of...
"I'm working on building a mechanical means to actuate a sliding window, but I'm having trouble with this specific bit. How can I attach the foo to the bar, without breaking the baz?"
The question would probably have been on topic, as the question is asking how to solve a specific problem. A question like that can be answered in a paragraph or two, and has a limited number of solutions. Asking the community to design a complex; potentially patentable, product, could lead to thousands of "correct" answers. The OP would then use some criteria (which is only known to them), to select the "right" answer. In the game of StackExchange, this is an unfair question.
This site is a game. You score points by asking good questions, and providing good answers. You get a bonus if your answer is correct. When you ask a question, you're setting the rules for that round of the game. When you ask questions with this in mind "so that I could select the design most likely to work in this situation.", you're asking people to play a game where the rules can constantly change. They call this game "I win", and only the winner likes to play that type of game. – Tester101♦ Aug 28 at 16:02
When asking a good question, you should not only include the rules but also the winning criteria. As @ChrisF points out. If you're having trouble defining the rules or winning criteria, there are usually folks in chat willing to help. While chat might not be the most "permanent" place to ask, it's often a good place to get suggestions on how to narrow and focus a question.