This isn't a rant...I don't feel like rants are useful topics and that they're not what you're looking for. Here are some negative things that I've noticed so far, and I hope others add to this list.
-No pages on topics. This makes for a potentially extremely long post that is hard to interpret.
-Direct replies being stored under posts. This undermines the use of quoting and seems like it would interfere with discussion.
-Hashtags. Yes, this allows for different "forums" to see interrelated topics, but never really seemed like an issue. In addition, it just looks unprofessional, childish, and lame.
-Small avatars, no title bar. Both features in .old served to give a sense of community. I'm not a fan of title elitism, but it was a fairly reliable system.
-Boring/generic color scheme and interface
-I love the new background, but what's the point of using it if you're just going to cover it up with a white to grey gradient border all around the forum window? In addition, why use such clashing colors?
-White background. This ought to go without saying, but it sucks.
The new site is cool. It's modern and trendy and that's what Bungie needs when Destiny hits. It might be nitpicking but it just feels like the community aspect has diminished.