The main thing I miss is "It's All Text!", which the creator / maintainer indicated was impossible to port away from Legacy, in no small part due to the security aspects of it.

I really, truly, hate inputting anything more than a couple of lines in to websites. Worse when you're fighting WYSIWYG editors, fixed box widths. Even worse than that is when you spend 20 minutes filling in the box, hit submit and find your session timed out.

"It's All Text" allowed me to just click an "edit" button beside the text box, go edit the thing in my favourite text editor, hit save and then go click submit in the browser. If the site screwed up somehow, the text was still open in my text editor.