Long overdue and welcome for publications wanting to create a brand identity and not feel subsumed by the Medium brand.
I hope greater flexibility with layouts will be the BIG change, as the examples of publications already using the beta system so far look... well.. pretty much like Blogger from 2009. Hopefully we have an option to retain the 'grid' layout of image-based blocks. Otherwise it looks very 'old-school blog' and not very 'modern-day website'.
I also hope bespoke drop-lettering is made easier. It's a real pain to have to do those manually every time. It would be great if you could upload an A-Z of images and just your have publication use those .PNG files as sources whenever you create a drop-letter.