Portability (size / weight)
If it weren't for the deal Gamestop offered on trading up from a DS Lite, I probably would have left the DSi alone. Sometimes, it's best to go with your gut. While not a huge divergence from the Lite, the DSi's main upgrades are of questionable value compared to the loss of the GBA slot. The cameras are a joke - they have a sensor quality akin to my first Casio digital camera back in 1998. The DSi store has, thus far, been a haven for bits of already released games (i.e. parts of Brain Age, Warioware, etc), crapware apps that would be fine if free but often cost $2 and up and, well, that's about it. Battery life seems significantly worse than the Lite, even with Wi-Fi turned off and it tends to not feel as comfortable in your hands as the Lite or even the Phat.
My review score only takes into account the hardware and software upgrades. I still think the DS platform has a strong library that continues to grow. Recommend you stay away and just buy (or hang onto) a DS Lite.