Although CNET did not review the Samsung UN55D6003, we did review the UND6400.
Comparing the two on Samsung's Web site, the main differences include 3D compatibility and styling. The less-expensive UN55D6003 is 2D-only and can't display 3D images (more info) and also has a thicker bezel around the edge of the screen. Comparing the two televisions' dimensions implies that the UN55D6003's bezel is 0.5 inch thicker than that of the UND6400.
The UN55D6003 also has a "clear motion rate" of 240, as opposed to 480 for UND6400. These numbers are derived from the TVs' refresh rate and as such should have minimal impact on picture quality. While we usually don't put much stock in contrast ratio specifications, it's worth noting that the UN55D6003 is rated at 4,000,000:1 compared with the UND6400's 5,000,000:1, which might mean the UN55D6003 has slightly worse black-level performance.
Finally the UN55D6003 has fewer HDMI connections (two as opposed to four), doesn't include a Wi-Fi dongle, and is colored "rose black."
As far as we can tell there are no other major differences between the two series. For more information, check out our full review of the Samsung UND6400 series.