For me I think it's a big step backwards in a lot of the design. It's suffering the same fate as the reddit redesign.
This may be covered by the Css upgrades on mobile, but so far for me:
-Everything is too big. The mobile screen is taken up by one or two topics at most, and I end up scrolling and scrolling just to see the same info i used to see all at once. Too much work for the user.
-The type is also giant, and making what should be one one line break out over two or three lines (i.e. The site nav text links under the "reply" title on this page)
-Last size thing: the buttons and icons are also enormous. (ie the construct logo in the top nav should be half the size, and the nav bar height with it.)
-Everything is important. There is no design hierarchy. Every element has a bright colour, often competing, and I have no indication of where to look. (i.e. The formatting buttons above this text box)
-The scrolling-boxes, like topic review, get in the way of the constant page scrolling I'm now doing (because everything is too big). When I try to scroll up on this Post Reply page, for example, half the time I end up just scrolling the topic review box instead.
I hope this is a work in progress, personally. It's falling for so many of the modern web design traps, and it's particularly sad to see that when the last forum design was so good.