I primarily use default formatting and basic Textile headers on my wiki pages to avoid issues with formatting. Unfortunately, in the new format, I can't distinguish a difference between Header 3 (h3.) and Header 4 (h4.). It seems worth reporting as a bug because there is a notable difference between Header 4 formatting in the preview window (bold with a horizontal rule beneath, which I like) and the same formatting after publishing (Header 3 and 4 being the same large, unbolded text with no horizontal rule beneath).
Cheers, and keep up the good work.