In terms of the hardware, they noticed that the screen was significantly brighter; the PSP Brite's highest brightness setting not plugged in to an outlet is noticeably brighter than the PSP Slim & Lite's brightest setting with the PSP plugged in. Unfortunately, they did not test the PSP Brite's performance in the sun, which was something we were looking forward to finding out. The battery life was then tested, and they clocked the PSP Brite's battery life at 5 and a half hours -- exactly the same as its predecessor.
The PSP also came equipped with the latest firmware. Firmware 4.20 appears to bring only minor updates -- there is a new "USB Auto-Connect" feature, which will allow the PSP to automatically enter USB mode at any time (except during gameplay) when it becomes connected. They also noticed a "Flicker Reduction" option, but nothing else noteworthy. For their full preview of the PSP Brite, check out Eurogamer.
[Thanks, Zach M.!]