It's been less than ten minutes since I've been on this page and already I have a dire need:
A checklist on: how do I determine whether a question fits on English Language & Usage or on English Language Learners?
I think I'll post this as an answer, because some of the opinions expressed in the comments seem so WRONG to me, not just in and of themselves, but also as a basis for a site.
First of all, we CANNOT define this site as "questions that can be answered by a native speaker with no particular expertise", because another way to describe those questions is bikeshedding: questions that everyone has an opinion on, and that people are capable of debating to death even though it's a very minor point. If all you can say about a question is "I'm a native speaker and it sounds better this way", without being able to explain why, then who's to say whether you're correct, or whether that other answer that says "I'm a native speaker and it sounds better that way" is more correct?1
The heart of Stackexchange is expert answers. If this site isn't about expert answers, then we might as well close our doors and go home.
1 Also, people tend to learn a rule better if they understand the reasoning behind it. I know that if I can explain the "why" behind a particular facet of language to my boss (who is not a native speaker), he is much more likely to remember it the next time. Otherwise, I end up correcting the same error over and over again. In a sense, if we leave out the "why", every question becomes "too localized".
Instead, we have to define ourselves in terms of the questions that can be asked. Waiwai's answer is on the right track, with the distinction between native and non-native speakers. But that's a bit too, well, ad hominem. Instead, I'd phrase it as:
If your question is about the English language, but is unlikely to ever occur to a native speaker (except in the context of teaching English as a foreign language), you should ask it here.
So, "I need a word for [concept] because I want to construct a sentence a particular way" belongs on ELU, but "My native language has a word for [concept], is there an equivalent word in English" belongs on ELL. "Is there a dialect where 'on yesterday' is idiomatic" goes on ELU, but "Why do we use 'on' with Monday but not with yesterday" goes on ELL. "In dialects that don't pronounce word-initial 'h', how do they distinguish 'ate' and 'hate'" goes on ELU, but "My native language doesn't have 'h', how do I remember the difference between 'ate' and 'hate'" goes on ELL. And so forth.
This is something that will very likely take a few weeks at the very least to figure out, as ELL and EL&U figure out their boundaries, so to speak. Thus, very rough guidance follows:
If you natively speak English or are fluent in it, it is more than likely your question belongs on EL&U.
If you are not yet fluent in English, or your question is one most fluent speakers wouldn't have, ELL is more likely to be the appropriate place.
Please go ahead and ask your questions. If you post on ELL and don't get answers, drop in the EL&U chat room and see if it is maybe a bit more advanced than you think. If you post on EL&U, and it's not right for us, we'll probably close it but point you back here to ELL, and you're more than welcome to repost it here on ELL.
We brought this up in a proposal discussion. I'll copy my suggestions here.
ELL is intended to be geared toward the needs of both people learning English and people teaching English. I feel these general topics encompass many of those aspects:
EL&U is geared toward expert and advanced level question about the English language and how it is used. I feel that these general topics represent the level of questions that are desired on that site:
How about this:
If you have something to ask about a certain linguistic element for your own practical use, ask on ELL;
If you are not mainly concerned with using the element yourself, but instead you would like to know more about it, such as how it came to be, whether there is a pattern behind what you observe, etc., ask on EL&U.
This means that practical stylistic advice should also go on ELL, even though it will often be native speakers who ask such things. Of course the difference is only gradual, but questions should be filed according to their main focus.
I've mulled this one over for a long time now, and I've realized that the right place to ask not only depends on the nature of the question, but sometimes it depends on the nature of the answer sought.
If you'd like someone with a strong background in linguistics to give an explanation that goes something like this:
These two accusatory words come from two separate protolanguage roots. If the first is a preposition, then it must have a homophonous derived plural noun. Predicate count nouns with an indefinite subject are normally singular, with an indefinite article, unless the speaker wishes to emphasize the non-singularity of the subject.
then ELU would be the place to get an answer like that.
If instead, you're wanting someone to explain:
This expression basically means that two or more people are in strong disagreement with each other.
No, that's not the way a native speaker would normally say it. The order of the last two words are usually reversed.
then you should probably ask on ELL.
This litmus test may not be applicable in all cases, but I think it can be useful at times.
I like waiwai933's answer, but I feel it isn't refined enough. It focuses on the person asking the question. I feel the criteria should be based on the question itself.
I propose the following
For example: the etymology of words can be a useful tool in understanding their definition. But you don't need to know the etymology of English words to be fluent in English. Therefore, etymology questions should be directed to ELU.