Noticing (like many other editors) the fact that the Preview button is only available in the ‘Save changes’ popup, I’ve got a wish for this popup which already annoyed me a lot with the Visual Editor: I’d like it to appear at the top of the editor, toggling quickly, just like the Search & replace and the Special chars toolbars, and with a more general text such as ‘Finalize’. The current system, while being very counter-intuitive, makes the popup long to appear (one second, which is very long when you just want to change some typo or links), and it implies summarizing the changes without being able to check them in the editor because it is disabled (so it needs a big intellectual effort if we just made an action which is not common for us). Having the ‘Preview’ and ‘Review changes’ buttons in this toolbar would then be quite acceptable.
Another suggestion: on my desktop computer, I’ve got two monitors each using a resolution of 1920×1080. Seeing the preview in the full viewport without even having the contents on the borders that reminds what the readers will see (the project interface) is just waste. For my case, it would be really helpful to have the preview just next to the editor, I mean: the editor on the left, the preview (or the reviewing of the changes) on the right. The ‘Preview’ and ‘Review changes’ buttons could then be at the top of this panel on the right. We could have a button on the right of the main toolbar that would toggle this panel: a first click would show the panel on the right (taking half of the editor’s area), a second click would show the panel on the whole editor area, a third click would close the panel. Also, I’d love to have two buttons between the editor and the preview when they’re both shown, to get the scrolling of one panel into the other (this would need some algorithm to really have in one panel what you have in the other). Such an interface would be a strong improvement compared to the old editor (I could extend my browser window on my two screens to have the editor on one screen, the preview on the other), while currently it can still help to have a text editor next to MediaWiki’s editor just to be able to get this layout.
A third unrelated remark: I see, at the bottom of the editor, the article’s categories, which are semi-translucent/disabled; and they’re not displayed in the preview (just like the table of contents) while, to me, they should be.
Besides, I really like the look & feel of the editor; I’m keeping it and hoping it’ll improve quickly (because Wikimedia projects really deserve much better editors).