As discussed at this thread, a very important feature of the classic Wikitext editor is the possibility to see the Preview results and the wikitext edit box in the same page at the same time.
The Visual Editor obviously doesn't need this; but the 2017 Wikitext Editor has copied the flow of the "Review your changes" panel, obscuring the edit box below it, which is completely inadequate for wikitext - as it makes it impossible to compare the code and the result it produces. (Heck, even this very Phabricator edit box gets it right, showing an instant preview of this code I'm writting below it). Please, PLEASE change the layout of the Preview panel so that it shows above, below or alongside the wikitext code, instead of floating and obscuring it.
Related feature request: T153306 - A more accesible Preview button.