I've accepted thomij's answer for this question, but I don't want this dialog to die down, as there's still a bit to be hammered out. I'm going to try to incorporate the highlights from my comments above as well.
Where the "enough" line should be drawn is a matter of debate, but it will be objective once it has been decided and can clearly be explained in the homework policy.
I suggest that we choose one of these two goals:
- "Encourage" the poster to demonstrate a commitment to solving the problem on their own
- Get enough information to be able to answer the question effectively
I think we need both goals, honestly. We'd like people to show an effort so we don't become a homework service (this has happened to another high profile SE site, so it's not just an empty consideration), but we'd also like to understand where the OP has gone wrong, which speaks to your second point.
I've said this in the past, but I have a hard time believing that someone has "nothing" to go on, so even saying "well, I think the pressure of a fluid is found by PV = nRT, so if I have the temperature" it's easier to let them know they are way off base and why, rather than guessing about what they know or don't know. This works for the answers as well, since you could start all over from the very beginning, but the OP might only have one inconsistency in their knowledge that's further up in the hierarchy.
For the second option, we would have to let those who answer the questions decide how much information is enough. Obviously zero effort would not be acceptable, but for something like "I know it has something to do with moles," rather than voting to close, it would be better to just ask them to elaborate if you are interested in answering the question and they don't provide enough detail to do so.
You have brought up an important point, that you can prompt the user for more information. This is a great idea, and I encourage anyone who comes along to do so. Even if we end up closing the question, the user can still edit the details in and it can be reopened.
I would welcome people to vote to reopen and flag the post at the same time (which can be accomplished through using the "other" option) so that we don't have to accrue the full number of votes to reopen. We're not trying to prevent homework assignments (though we secretly wish that people would ask more conceptual questions instead!), but it's important to incorporate people's common misconceptions into the body of their question in case users come along later with the same problem. Bonus if the OP puts a good descriptive title on the question, but I have to choose my battles. That way the question can be found through search.
Much of the above comes from my comments on the other answer, so here's a bit of an expansion on those and something of a recap.
My feeling at this point is to say that anything better than a verbatim copy and paste of a question is fair game, with the provision that our more experienced users will prompt the user for more information. This is to the questioner's benefit, really, not a chance for us to be curmudgeons who have paid our dues already. I am, though, inclined to be a little less lenient with users that have been around for a while and who habitually engage in this behavior.
In the past, in certain borderline cases, I have given users a few hours to come back before closing, which we can do for all but the most egregious cases. Of course, other issues may crop up and can be handled on a case-by-case basis. If you think that a question has been left open or closed unfairly, speak up in the comments and/or bring it to meta if you start to see a pattern. I'm not inclined to discuss each and every question that's closed on meta, but general patterns can and should be brought here.
Based on the discussion in the comments below, I have a new suggestion. What if we were to adopt a policy of editing specific questions that showed no evidence of work into more general "how do I?" questions?
As mentioned above, I would love to have a site with conceptual questions that can be applied to any homework problem, but some people do learn better with examples.Frankly we don't want to shut everyone with a homework issue out since a lot of people are turning to the internet these days, and we'd like to retain some of the good new talent out there for when all of us get too old.
So, if you see something that would do better as a conceptual question (this particularly applies to those mini-essay type homework problems), give it an edit with every attempt to keep the original intent of the question intact.
Along the same lines, if you see a title like "chemistry problem 27" or "mols question", jazz it up a little bit. Bonus points if you can formulate it into an actual question in the title, but sometimes that makes the grammar awkward.
As usual, I want to hear some opinions on this in the comments, so don't be shy. Aspects of this that you'd like to refine further can be made into their own meta questions.