Rebecca Chernoff asked: New users often are not accustomed to the Stack Exchange system, and sometimes struggle to present themselves properly, either in the way they use the site or their attitude. How willing are you to work with "problematic" users, and at what point do you decide that someone isn't worth the effort?
toscho answered: Oh, this a hard question. :) Usually, I a comments to help these users learning the system. For example: New users misuse the answer field for updates to their questions – I explain then that questions can be updated directly.
toscho continued: I give up when someone violates basic etiquette or refuses to learn.
Jared answered: I don't think anyone is 'not worth the effort', people who want to get their questions answered will likely be accepting of advice on how to improve their question. If not and they are rude and do not want to learn, then they probably will not stick around anyways.
Brian Fegter answered: The first thing I look for in a question is their 'tone' as it were. Are they presenting a legitimate question because they've tried and need help? Or, are they trying to get someone else to do the hard work for them? I think the answer gives us a good gauge on their motivation and if investing my time into them is good for the community and for them as well. If they are not abusing the system, but don't have the SE semantics right, I'm happy to help them out.
Brian Fegter continued: Problematic also doesn't necessarily mean abusive. If problematic means novice issues, I was a novice to WPSE one time and had help.
Rebecca Chernoff explained: The idea there being users who treat the site like a normal forum, replying to other answers in a new answer, consistently asking questions that don't fit the format, etc.
Brian Fegter responded: If that's the case, then give time for the user to learn the system and help them along the way. I haven't really seen a case where someone doesn't want to do things the right way.