Many of our users will not have extensive written English skills. We can't give a lot of feedback about the edits we make in the description box, but those edits might make it difficult for the OP to understand what the question has become.
How much editing is acceptable?
I think ELL is a special case where even trivial edits should be encouraged if they're correcting obvious grammatical errors that wouldn't be made by a competent native speaker.
Many questioners here will make minor errors (wrong verb tense, missing article, etc.) that don't detract from legibility, and would therefore often be left "as is" on other SE sites. If other non-native speakers do recognise the errors, they may (subconsciously, at least) think the worse of ELL. If they don't, the site risks causing people to acquire or retain bad habits.
Having said that, I wouldn't particularly want to encourage edits that just "improve" poor phrasing, etc. Not that I think there's necessarily anything wrong with doing that in many cases; I just don't think it should be explicitly either encouraged or discouraged any more than on other SE sites.
You should always edit to improve a post wherever it makes it clearer or more syntactically correct. But don't make it abusive. It's generally considered poor etiquette to play the staunch editor who "fixes" every post with nothing but minor pedantic changes. But if you can improve the post substantively, you should; it can only help the site.
Keep in mind that the life cyle of a question goes well beyond helping the original author. One of your goals for this site should be to keep improving the content for those who come after. Folks will find your site through search, and a good end-user experience is the best way to keep those users and make this site flourish.
Besides, what better way is there to learn English than to have someone correct your errors, especially in the supportive context of an "English Language Learners" site?
I think that you should always correct non-trival errors. Especially with a site designed to assist people learning English. For instance, if a person were to come along and think something was proper, when it wasn't, that would debilitate their learning. As said by Robert, though; Do not make trivial edits, just as you wouldn't on Area 51, or any other Stack Exchange site.
Try to make the post substantively better when you edit, not just change a single character. Tiny, trivial edits are discouraged.
I'd err on the side of not editing, but pointing out an infelicity in a comment. In that way, a correction can help not only the OP, but other, later, users of the site who may not realize that what was originally written was problematic.
