Thanks for the work updating the forums. It's a tough call changing the look of a community space.
My personal opinion is that I like a "clean" look more than poofy icons, but apparently it's all about colorful icons with the damn kids on my lawn these days.
Keeping vertical spacing small to permit more content on the screen is worth the effort, and that's one gripe I have with big poofy icons in the "modern" interface.
Functionality is good. For example, in the classic view the "quick reply" textbox is not active so you have to actually use the reply to thread or reply to comment buttons instead of just starting to type in the text box at the bottom of the thread. In a world where whoever dies with the fewest button pushes wins, maybe activate the quick reply box? It's already there in the classic version, it just doesn't work.