What makes a good on-topic question?
I envision this site to be a place for people to bring their research problems (from any field) that involve signal/image processing and get solutions or suggestions for them. There are several inter-disciplinary questions and the recent one on finding stomata in plant microscopy images comes to mind as a good example. The question on river detection in text is yet another great example of someone bringing their problem from a completely different field and asking the experts here if it could be done (and it got great answers!).
Another good use of this site is in understanding algorithms used in certain applications (e.g. algorithm used in photoshop's B & W adjustment layer and explanations on fundamental but often misunderstood concepts (e.g., How are time resolution and frequency bandwidth related). I really, really dislike the "most lucid explanation to teach your grandmother to use wavelets" kind of questions and I hope we don't see them very often. However, that's a different beast to tackle.
What should be our policy?
Ideally, algorithms, ideas and approaches should be first and foremost and not code in language X. I mean, the answerer can use whatever language they feel they're comfortable in (e.g., the river detection question had answers in MATLAB, python and Mathematica), but the focus of the question should not be an explicit "give be the codez in X". Questions like show me how in any language and I'll work it out myself are also most welcome here.
Explicit requests for code in language X should be off-topic. The primary reason for not allowing questions that simply want code in X is because then we'll get overrun by which ever language is most commonly used in the particular field, and other folks who are more inclined towards theory and algorithm development part of it might be put off and leave. I can easily see MATLAB/openCV questions just flooding us if we opened the gates (in fact, we get a few such questions which you don't see because they are either promptly migrated or nuked right here).
Can we have the best of both worlds?
Coming to the question you linked to (electrophoresis gel), I in fact did comment on it and asked the OP if he was looking for code or approaches. He did seem pretty clueless, and looked like he wanted some code to solve his issue right away, so I left it at that. However, barring that, it fits very well with our site's scope and is a good example of the use of image processing in diverse fields. Ultimately, diversity is the key to our growth.
So I decided to try something different. I requested that the question be left on Stack Overflow for a couple of days (a moderator did talk to me about your flag) and once the code part has been solved, I will request migration here, where users can provide an explanation of how to go about it in general. This way, the OP gets his problem solved, and we have a good resource for our site. I realize that our users might not have gotten the head start that Stack Overflow users would've gotten in terms of votes, and so I'm willing to put a bounty on every such question that I request for migration (only new questions), and does not have an already reasonable answer.
I do not know if this will work or if it's a good idea, but I'm willing to give it a try. Since this was less than a day ago, I didn't have the time to bring it here as a meta post, but it's good that you did! I'd appreciate feedback on this idea.
A final word on migrations...
Please do not request for migration of old questions unless if it's really, really off-topic on Stack Overflow (or elsewhere) and a very good question for our site. Borderline and meager questions should be left alone where ever they are. If the OP of the post is not around anymore (greyed out) or hasn't visited in 2 years, please leave them where they are. If there's an old and interesting question but not migration worthy in its current form (either because of poor details in the question or crappy answers), please just re-ask the question here in a better form and in a manner that makes it on-topic for us.