I'm not too keen on the menu box not having menu anymore. My biggest problem is that the Menu Box and the Layers Box are now Windows, which are set to "always be on top", and no way to change that. That becomes a real PITA when trying to edit an image in full screen mode. Next up is that many plug's are not compatible with this version. I have some plug-ins I've made since 1.x version of gimp and they have always been able to be converted to recent, until now.
I'll stick with older gimp versions until the gimp dev's can tackle these few things
New gimp does have some nice features, but some are just in the way