What exactly has changed or is better by the new Moebooru ?

Very little end user changes.

We have opengraph support on the images. (fb barfs on ssl though?)
Updated to a much more recent version of rails. (yay security!)
There's localized interfaces, but it is currently disabled because the UI would randomly change to one of the languages we translated to.
A news ticker (from danbooru)

I hope that answers the question.