When I make an edit, I almost always hit the review changes button to make sure I haven't accidentally changed anything I didn't mean to (e.g. by accidentally typing a random keyboard letter in the middle of the document while scrolling past it) . The visual wikitext editor puts this feature, and the Preview button, in a separate dialog that only appears after hitting Publish.
For me, this is a very unintuitive UI flow. I don't want to "publish" my edit until after verifying that it's correct and safe. When I'm done changing whatever I want to change, my first instinct is to look around for the diff and preview buttons, and to avoid going anywhere near "Publish" until I've double-checked my work. If a change is only half-done and I want to look at the preview to to see if I used a template correctly, it feels dangerous and uncomfortable to click "publish" with the editor full of not-ready-to-publish work.
The old wikitext editor has separate preview and changes buttons next to the publish button, instead of requiring the user to click the publish button first. Please add similar buttons next to the publish button in the visual editor, or at least a drop-down menu containing them.