> 5. Everything degrades gracefully when JavaScript is off. For example, preview-as-you-type becomes a "Preview" button if JavaScript is disabled.
Especially those sound great!
Some feedback from poking around the demo a bit (on Desktop):
* It has a lot of whitespace, to the point that it forces a lot of scrolling. The "front page" with the list of existing boards is particularly bad, with only 4 cards fitting on my 1080p screen, where a typical phpBB fits 10 or more. And the layout of the cards is still quite busy, with 5 text styles close to each other. Your demo hasn't got any posts with long texts, so it is hard to judge how big an issue this is in the threads themselves. Still, on a desktop you have the space to the side, maybe put the name besides the post instead of on top?
* A lot of elements have dynamic positions (e.g. the date, the author) which makes it quite hard to quickly scan the overviews for them.
* Image support? Something a lot of forums have issues with, which leads to various hacks, images not visible to visitors, and often broken images from external hosts. This is an area where you easily can be better than many existing boards.