T9905 mentions a couple times that the remarkup previews disappeared from maniphest with the applicationeditor changes.
I've been scared and holding off on updating because of that.
I just now decided to try it on this install and can see previews as far as I can tell. If I got to create a task from the top right, I see a description preview as I type. Likewise, if I start editing an existing task, the preview changes appropriately.
So, I just wanted to figure out what exactly people were upset about so I know what to expect when I unleash this on my users.
It was really touch and go there for a while, but I gritted my teeth, furrowed my brow, and clicked "Accept" on the magical things @epriestley had typed and it all worked out.