What Makes G1000 NXi Different From G1000?
The official Garmin announcement lists all new features.

The new hardware features are:


faster processors resulting in faster start-up, map rendering, etc.
more modern animations
a new design with better readability
LED back-lighting (increased display brightness, clarity and better dimming)
lower power consumption


The new software features are:


wireless connectivity (database updates, flight plan transfer and information shared with a mobile device running the Garmin Pilot or ForeFlight apps)
flight path for visual approaches (both for visualization and auto-pilot)
full map overlay on the HSI on the PFD
SurfaceWatch (similar to a RAAS)
full ADS-B compatibility


Further features listed are:


  
  Advanced Doppler weather radar capabilities, including ground clutter suppression and turbulence detection as options.
  COMM frequency identification displays the station ID, as well as frequency type.
  Display of sectional charts, as well as IFR low/high enroute charts on the MFD.
  NEXRAD weather radar imagery can be overlaid on the moving map and animated on the MFD, as well as the HSI map.
  Vertical Situation Display (VSD) incorporates terrain profile view on the MFD, taking into consideration the active flight plan, altitude constraints and winds aloft.
  Pilots can preview departure and arrival procedures on the MFD prior to loading and activating the procedures.
  Pilots can optionally overlay European Visual Reporting Points (VRPs) on the moving map, as VRPs are standard within the navigation database.
  Three-color terrain shading incorporates green, yellow and red contouring, which depicts the aircraft is 2,000, 1,000 and 100 feet above ground level (AGL) respectively.
  Aircraft maintenance personnel experience faster system software upgrades and streamlined equipment replacement.