I...just stated the highlights. :/ I suppose you could add that it can now play PERFECTLY LEGITIMATE backups of games without a modchip, but that's more of an extension of what can currently be done than anything else. The entire system has basically been owned at this point.
The ability to downgrade firmware means that no matter what Nintendo does they're not even going to be able to stop people without modchips. Of course, you can't exactly rely on being able to prepare a special battery to put the system into service mode, but there's always going to be backdoors, especially when it comes to loading saved games and such. Nintendo, like Sony with the PSP, isn't going to be able to keep the system locked down forever. What's worse is that Nintendo seems to be a lot slower at patching holes like that than Sony is. Sony is known to quickly patch their firmware whenever a save game exploit comes out. The Twilight Hack was open for quite some time before Nintendo finally closed it in an effective way and, apparently, homebrewers have already found numerous other holes in other games.
I'm not too worried, really. Nintendo's idea of patching security holes is just plain laughable. When Sony stops a saved game exploit, they stop it. Nintendo's idea of stopping the Twilight Hack in the v3.3 update was to create such a specific "fix" that it took no time at all for people to find a way around it and rework the exact same exploit. Their idea of stopping the Custom IOS was to create their own IOS254 (the tag that cIOS installs itself under). What's to stop the homebrew community from using the IOS253 tag (or something similar) instead? Nintendo's solutions are completely ineffective. They're better off not bothering. After all, Sony's doing a much better job at patching their system and they can't keep people out.