The HX929 is an LED-backlit LCD, while the HX729 is an LED edge-lit LCD, meaning the XBR contains a full array of LEDs behind the LCD panel, enabling a feature called local dimming. The TV is able to independently control clusters of LEDs grouped into zones dynamically based on on-screen content. This gives you the best available backlight homogeneity, especially in larger (e.g. 65") screen sizes, better brights and darks, and the deepest blacks available on LCD televisions.
The HX729 only features LEDs around the edges of the LCD panel and as such cannot feature real local dimming, although it does feature a poor man's version of it that in largely ineffective compared to the real thing.
ht Panasonic TC-P60ZT60, Monitor Audio: Silver RX6, RX Centre, Radius 90HD; Martinlogan Dynamo 700, Marantz SR5006, PS3, Oppo BDP-103D
2ch Marantz PM8004, Sony BDP-S1000ES, JVC T-X3 tuner, Monitor Audio Silver RX1, REL T3, Apple TV, Peachtree Audio DAC•iT
lr Panasonic 50ST60, Sony BDP-S5100