Posting: when posting, the default forum should be the one I'm in when selecting "new post". I shouldn't have to select it every time.
Quoting: quotes are awful. The giant double quote looks silly and takes up too much space. There is no mark for the end of the quote, and quoted text looks just like other text. There is no way to automatically quote the message I'm replying to.
Back navigation: it's broken - previous pages must be reloaded all the time. With pages of 1.5MB it makes using a mobile devices nearly impossible
Searching: there doesn't seem to be any way to restrict searches to a particular forum, author, date range, etc.