The real distinction between ELU and ELL isn't really the questions (easy vs hard) - it's the type of answers you're looking for.
The respective tours say that ELL is for "speakers of other languages learning English" while ELU is for "linguists, etymologists, and serious English language enthusiasts". In theory, you join the community that better describes you. In practice, there are people who have joined both communities and participate actively in both.
ELL includes questions about the mechanics of the language, asking questions like which tense one should use in a particular example. Both questions and answers might include linguistics terms such as object complement to more precisely express what they're trying to say. It's not a matter of ELL being 'simple'. I'd consider myself an English enthusiast. It's my primary language, one I picked up as a baby, but my formal training in the technical matters of the language extends only to what some might call 'grade school'. When discussing parts of speech, for example, I tend to find it easier to say whether a phrase is idiomatic, or what the phrase means, than to tell you whether it is an object complement and, if so, what kind it is (I'm getting a little better after spending time on both sites, though).
ELU is not specifically about the mechanics of the language. Ideally, we'd assume that the community is well-versed at parsing sentences and has a basic grasp of common idioms. We'll look at the finer nuances behind similar terms, pick the 'best word' for an occasion, or more generally ask the 'why' questions, more so than the 'how' or even the 'what' questions. It's not that the 'how' or 'what' questions are necessarily off-topic - the site explicitly includes linguists, after all, but if you are already familiar with the language and wish to probe further, you'd probably ask your question on ELU instead of ELL.
Having said that, I have found some of the most interesting questions on ELU coming from those who aren't native English speakers (but don't call this group 'natives' as others have done - to native speakers, 'natives' tends to refer to totally different groups of people than what was probably intended). Most of us at ELU welcome people from all stages of language mastery, the most important caveat probably being that we need to be able to converse intelligibly with one another fairly easily in English.
In relation to your 'salt' example: as noted in comments to the question, we at ELU enjoy questions about English versions of foreign-language idioms. They are welcome, so long as we have sufficient background to understand (in English) what the idiom is trying to express. The question should not assume that any potential answerer knows anything about the non-English language.