The redesigned backend interface looks just sexy and is really fun! Tons of tiny details. Go, download NB and try it out. Just for the UI experience look to the usergroup mapper. The user management is unique, and integrates instead of syncs. Last but not least there are the forum converters. Converters, IMO with the best converter UI I look to date in the J! market.
From the developer point of view, the codebase was reduced drastic compared to the old 0.5 NinjaBoard code. Built from scratch and on top of Nooku Framework, it is a pleasure to read NB code. Far away from spaghetti code like in other forum solutions for J!.
For a designer, NB is also a dream. Chameleon is not just another marketing buzzword. It is the new auto-skinning template engine that really saves you time, looks good and just works on your site - with any template for joomla.
And with the coming Anahita, JomSocial and CB integrations, things can only get better!