Arstechnica has posted a comprehensive five page review of the DS Lite, with a large portion dedicated to comparisons with the DS Phat. This isn't the first review of the DS Lite, but it could possibly be the most well thought out. Here are the key points of the review:
Greater viewing angle - "it's now possible to watch someone else playing the system and actually see the screen"
Brighter - "the upgraded screen may be worth the price of an upgrade alone"
Dead pixels - "my unit has a stuck pixel on the top screen"
Flimsy? - "the bottom screen seems to be floating in the system case, not held down tight like the DS [Phat]"
Lighter - "the system is lighter, making it easier for me to cradle it using the tips of my fingers"
Buttons - "The D-pad is a touch smaller... and a bit less satisfying in the click department" although "the buttons on both DS products are a little bit on the small side"
Central microphone "makes games that use voice recognition easier to play."
Six hours usually, although "it's not hard to get eight hours of battery life as long as you don't continually keep the system on its brightest setting."
The overall feeling from the review is that if you're a DS virgin and have been mulling a purchase, now is the time to jump in. If, however, you're a DS Phat owner and haven't decided whether it's worth trading in for the updated hardware, the suggestion from this review is go for it. "If you're a casual gamer or a hardcore fan who takes your portable everywhere, you'll like the way your games play and look on the DS Lite".