I have noticed that on ELL many people give answers using rare/difficult words and constructions. Some write in 'an English' not all learners can yet understand. I know I can, although at times I have to search for a particular word in the dictionaries. Not all learners can understand what others have written. Why not use Simple English to give answers?
There are two sides to this argument.
On one hand, people who answer on ELL should be aware that the person asking a question may have limited English skills. We should conscientiously try to not overwhelm the person with fancy words and complex sentences.
On the other hand, if we simplify things too much, how will you expand your vocabulary? I won’t apologize on behalf of the community if you have had to look up words in a dictionary – that’s part of learning the language!
There is a risk in eliminating fancier words or replacing them with easier synonyms. If everyone did that in every answer, the site might start to read as though everything was written in “baby talk.” Fancier words are often more precise and convey more meaning; simpler words can be misconstrued as rude or belittling. For example, I’m more comfortable with saying that you “might have limited English skills” than saying that you’re “not good at English” – the version that is easier to understand is also less polite! Also, did I really need to say “conscientiously”?
We should conscientiously try to not overwhelm the person with fancy words and complex sentences.
I could leave that word out for the sake of simplicity, but the sentence would lose a little of its meaning. Conscientious means:
wishing to do what is right, especially to do one's work well and thoroughly (NOAD)
So that word adds a little bit of meaning to the sentence. I think it’s better to leave it in.
I agree with J.R.'s answer, but I also want to add some things.
I just recently started answering questions here, after having one of my answers get migrated from ELU. I'm a native speaker, so I was initially a little concerned that my writing may be too complex for some learners to understand.
The thing is, it's not easy for me to determine which words are "too difficult" or when my sentence structure is "too complex". I also don't like to change what I've written after I found the way I want it to read.
After finding some of the linked posts here on meta, I decided to just write things normally, without trying to "dumb it down". If you don't understand what I'm saying, just ask! Most people will be happy to explain what they mean (just don't overdo it, see what you can figure out on your own first).
I'd also like to point out that Macs have a great feature: three finger tap on a word to see its definition. (This post explains how to enable/disable the three finger tap dictionary and this post explains how to change the dictionary languages.)