GBA was attacked by the users and the media alike (minus paid ones...) for the poor visibility of the screen that is not really suitable for high colour displays. According to Nintendo the device has been designed strictly for outdoor use under bright natural sunlight. Since public awareness of eyestrain devices become pretty high, many people who would have bought it avoided it, or bought it with a few games and stopped using it. To prove Nintendo lied about the impossibility of a lit display (since PDAs already proved that being perfectly possible by that time), many hobbyists created lighting solutions that should have been used in the first place, but would have made the device much more expensive at the time it went to market...
SP is a redesigned gba, what is changed is finally proper lighting and visibility of the display. Haven't tried it yet, so don't blame me if it is still not really good!
Test for yourself before buying and test in your hands not under the special lighting conditions used to fool customers at Nintendo boots placed in hypermarkets and stuff... They use the lighting type needed for gba display to be visible, but you can't do that under real usage conditions.
Also added is li-ion (?) rechargable batteries needed to support the higher power requirements, and a redesgined case needed for the new display and battery... and to say it is a "special edition" true Nintendo style instead of admitting they were cheap with the first design.
SP probably gives much needed life to the game scene, though an upgrade path would have been nice for existing users, since many of them may refuse to spend the launch price again for a product that could have been made properly for the first time.