CNET did not review the LG LM5800 series, but we did review a similar product, the LG LM6700 series.
According to LG's Web site, the two TVs have significantly different feature sets and styling (see below), but the main picture quality difference between the two comes down to backlight technology.
The LG LM6700 we reviewed has an edge-lit LED backlight equipped with local dimming (more info), which LG calls "LED Plus." The LM5800 does not. While normally this feature has a positive impact on black-level performance, and hence overall picture quality, during our review of the LM6700 we noted poor black levels despite the presence of local dimming. For that reason, we don't expect this difference to have much impact on the comparative picture quality of both of these LGs.
The LM5800 also has a lower contrast ratio specification than the LM6700, but given the 6700's mediocre black-level performance, we don't expect the LM5800 to perform much worse in this area. According to LG's specifications, the two TVs share largely identical picture quality characteristics otherwise, so we expect them to have similar picture quality overall.
The LM5800 lacks the LM6700's Smart TV Internet suite with built-in Wi-Fi and its motion-sensitive remote. The LM5800 cannot connect to the Internet, and offers a standard remote control.
Audio/video connectivity is another point of difference between the two TVs. The LM5800 has three HDMI jacks to the 6700's four, one USB port to the 6700's three, and two component-video inputs to the single component-video input on the 6700.
The styling of the two TVs is also different. The LM5800 has a black bezel around the screen, compared with the LM6700's silver. The 5800's bezel is also quite a bit wider than that of the 6700, resulting in a less-compact, modern appearance. The 5800 also lacks the 6700's distinctive stand.
The less-expensive LM5800 series is available in 42-, 47-, and 55-inch sizes, compared with 47 and 55 inches for the LM6700.
Otherwise the two 120Hz, 3D-compatible, edge-lit LED-based LCD TVs by LG are basically the same according to LG's Web site. For more information, check out the full review of the LG LM6700 series.