This time, Mario enters a dark cave to find some more power stars. In the cave he meets spiders, gets struck by falling boulders and meets a whole new - not hostile - but also not exactly friendly species. 
The new species is purposely not shown in this video though you can find out about them once this game is released!

new technical things include the crystals, smoothly transitioning music and obviously many unspoilered things.
this level also introduces a powerup new for 3D mario titles, be excited for it!