One thing to avoid is to change the intent of the question completely. Edits that are too radical or change the fundamental intent of the question should not be acceptable - some questions are just not repairable, and sometimes we have to ask new questions or risk completely changing the OP's question, which is something we don't want to do.
Sometimes, we can't fix a question and being it off of the on hold status. That is alright - we can't save everything.
Making some attempt to correct a question to make it fit is alright, but changing the question or adding details that only the OP would be able to confirm or know about is not a very good idea - the point of editing is to make adjustments, not to change things completely.
It is entirely possible a question will be on hold for a day or more or perhaps forever (especially if it is considered off topic). While it is not ideal, it is just the way it goes - we can't make everything work, as much as we would like to.
We are attempting to create a good solid base of awesome questions, and editing is part of that! But we also don't want to change the intent of the question or make it completely unrecognizable from what the OP was actually trying to ask (unless it is poorly written, etc).
This is one of those cases where we don't want to treat edits now any different than the edits we do later when the site is in public beta. We want good edits that improve questions, not change them or make assumptions about things that we really have no way of knowing.