It isn't obvious to all users that clicking on the warning about Wikitext will make it disappear.
For example, PamD at en.wp said:
"I edited an article and found I'd got the popup saying I'd been using wiki markup, I should go into Edit Source, my edits wouldn't be saved, etc. I knew I'd input a pair of ''s at one time but had then remembered the error of my ways, removed them, and marked the title as italic in VE style, so was pretty confident I hadn't used any wiki markup and went ahead and saved the edit (difficult because the popup was sitting on top of the savepage box)."
The main issue there is Bug 52155, but after being pointed to that bug she replied:
"so if I'd clicked on the box it would have gone away... ? Not obvious!"
It would be good to add an X in the upper right corner to indicate that it can be closed. This wouldn't change the behaviour that clicking anywhere on the box dismisses it, just give a visual indication that it can be done.