Summary is that they are changing from an organic look with things spread out in somewhat hand-designed fashion into a more common style with everything across the top (and looking a lot like recent Google UIs, too).
I think there are pros and cons despite their simple claim that "Our new interface is simpler, cleaner and better." There were reasons for the old UI, I think. They weren't just drunk. However, it is yet another example of an oddball interface made to conform to common expectations.
Long ago, I worked on a WYSIWYG HTML editor that went through the same kind of evolution.
What's common often is what's easier, especially when you want to grow to new users.