First of all, new games! We might not have seen any yet, but the DSi games might very well be the best thing ever. And, when you look at the support the DS got from its release up until now (and beyond), from both Nintendo and third parties, you have to feel good about the potential for a strong DSi library of games. And that's not even considering how all of the new features will affect those games!
Speaking of new features, the Nintendo DSi might not have the best camera on the market, and putting AACs onto a SD card might be kind of annoying, but Nintendo is getting closer and closer to making their handheld the only gadget you need on you at all times, aside from maybe a cell phone. And, when you think about it, the camera on the Nintendo DSi has to rival that of the cameras on most cell phones, probably even performing better.
Then there's the onboard SD slot. My head spins at the possibilities of having an onboard SD slot and what the homebrew community is going to come up with. It's like Nintendo is recognizing the R4 is huge and everyone is pirating things onto the handheld, so they just threw their arms up and gave up. We imagine that Nintendo is thinking that if they're doing all of this stuff with the DS and DS Lite, they might as well include the SD slot on the Nintendo DSi and hope the homebrewers and pirates will see that as enough reason to pay for a new handheld.
Finally, the app store. How awesome is that? Seeing as how WiiWare has been a pretty decent service so far, we can only imagine the amazing titles we'll soon be able to download to our Nintendo DSi. And, for as cheap as it is to develop on the Wii, it's even cheaper to develop on the DS, so we expect a great flood of wonderful indie titles available for download.
The future is bright with the Nintendo DSi and I can't wait for it.