Proposal: English Language Learners
How shall we clearly define what is on-topic for this site and what is confusing enough that it should properly go to English Language and Usage?
The issue of forming a site exclusively around "beginners questions" is one of reciprocity. Most sites at least perport to be experts talking to experts and answering each other questions. They, in turn, attract a fair number of enthusiasts and beginning learners asking interesting (albeit, beginner) questions.
In contrast, an ESL site is setting itself up as a core group of teachers willing to help a site full of beginning learners. There will be little, if any, reciprocity.
English SE has not panned out as an "expert-level" site. There is very little pretense that this site is only for advanced users discussing on the "finer points" of the English language. I don't see any (good) questions in this proposal Definition that are not perfectly acceptable in English SE. The only questions being rejected by English SE are ones that should not be asked anywhere — https://area51.meta.stackexchange.com/a/5702/5.
I'm not seeing a case a separate site at all.
If the question is one where the main source of confusion comes from being a non-native speaker – that is, if the question is justifiably confusing to the O.P., but just about any native speaker could probably answer it without doing any research, simply by recalling basic lessons in middle school – then the question would be a better fit for this site than for EL&U. However, some users may not realize how basic their question is, so the tricky part becomes stating this such that it becomes obvious which site would be more apropos.
Maybe something along these lines might work:
If your question stems from not having the same level of familiarity with English as a native speaker, and most native speakers could answer your question based on first-hand knowledge, then it's probably a good fit for the ESL board.
However, if your question is more abstruse and less straightforward, if it might cause even a native speakers to ponder, and perhaps even prompt them to do some research, then EL&U might be the better place to ask the question.
I'm not really even 100% satisfied with that suggestion, but it might be useful enough that someone could refine it, or use it as a springboard to describe the difference even better.
Lets take the question "Then or Than, Which to use when comparing time?" This is clearly too basic for EL&U. Just check a dictionary and it will tell you that then and than are used in very different circumstances. (OK, it's been closed as a dupe, but the first 4 votes were general reference. Things have changed a lot over the last year and a quarter, and the dupe would be closed as GR these days.)
Is that enough for a person who is beginning to learn English? Someone who is not a serious (i.e. knowledgeable) English Language Enthusiast? I would say no. The definitions in a dictionary are separate, and don't compare the two words (because why would you? they only look alike). They don't help enough to aid remembering when to use then and when to use than.
An answer for someone beginning ESL, or even maybe someone who is intermediate, would explain how, even though the words sound similar, they are not used for the same thing. The answer would show how it is illogical to use than in place of then and vice versa.
General Reference for EL&U is not necessarily gen ref for this proposal.
So words that cause confusion for people learning English should be on topic.
I'm not convinced that 'clear definition' is called for.
In the first place, supposing it can be achieved, will it have any effect? Do the 'clear definitions' on ELU actually inhibit bad questioning? Perhaps they do, but the fact that this alternative site is proposed indicates that their effect is inadequate.
In the second place, is 'clear definition' desirable? It seems to me that clarity is achieved more through practice than through definition, and that whatever definitions are put up will yield eventually to actual usage.
I suspect that after a relatively brief shakedown cruise the best definition will be ostensive: "Browse around both sites and take your question to the site where you feel most comfortable - where the questions asked and the answers returned speak to your needs."
How can one draw a line between English as a Second Language and English for natives of English (or whatever one might define EL&U in comparison) when one knows that English speaking natives sometimes ask questions non natives who have actually learnt grammar wouldn't? Of course you can object that people don't always advertise their native language in their profile and one can't be sure. I agree to that.
I will take an example in the usage of the possessive where I have found natives of English have a far lesser knowledge than ESL learners and make more mistakes because ESL learners will be more careful and know where to look for the information when they hesitate.
As example questions I have looked for some that seem to have been asked by natives of English and haven't been closed as General Reference :
Admitting I'm right and these questions have been asked by natives of English, would the OPs have spontaneously posted on English as a Second Language ?
And wouldn't other English natives benefit from any answer on these particular points?