there is a lot of improvement hidden under the hood.
Pretty much what he said. Both Mavericks and its immediate predecessor have been more about under the hood rather than bells & whistles.
Which in my opinion is how it should be.... otherwise you end up like Microsoft with Windows 8 where you've evidently ploughed 80% of your resources into a fancy interface and only made rather modest changes under the hood.
One of the best behind the scenes changes for Mavericks comes for laptop users. Mavericks has fancy energy saving stuff going on behind the scenes ("app nap" etc), and for laptop users that means a genuine improvement in battery life.