My initial thoughts. This is a big change and gamble for Nintendo. Taking all the lessons they have learned in both the console and handheld market and putting them into one system is a pretty big and bold direction for Nintendo.
It allows Nintendo to focus on a single system for games and really emphasis what makes Nintendo different, there games and unique ways of playing. It also seems future proof in that the tablet part could be upgraded every 3-4 years just like an Apple ipad. We don't know if there is any hardware in the dock to help with upscaling and processing, but if there is, then the first upgrade for the system would probably just be to match what the dock does. It also means gamers could more affordably upgrade their dock every once in awhile.
If Nintendo has a winner with this system and it is popular then Nintendo could have created a sustainable infrastructure for upgradable hardware in both the portable and home market...and paying a slight premium for both experiences is still cheaper than buying two systems for both experiences.
Going with cards is interesting, because you wonder if Nintendo will continue to do downloaded games. This could be where Nintendo once again cheaps out, with a smaller hard drive. However, cards are durable easy to carry and retail and resale friendly so I am OK with that. The Dock may also have some additional hard drive space and maybe you can store games on both.
The bad is simple, this system is easy to copy and as already mentioned something similar already exists. This means Nintendo is going to need to push hard their new system with games, and boy did it appear that Nintendo was going to go big year one. I think Nintendo realizes year one is a make it or break it year for this system. And Year one looks to have Mario, Zelda, Mario Kart, and Splatoon right out of the gate. I don't mean all launch titles, but all available in December.
Next year could equally be big because we have no idea what Retro has been working on (unless it was Mario Kart.) And we could see even more games in 2018 because by then Nintendo will be in full support of only a single system. If Nintendo got it's out put to be 1 new game every 4 months and perhaps VC and Downloadable smaller games in the gaps, then it can support this system by themselves.
Finally question mark is the controller, I am not sure I like the button placements, and I am hoping they created a way to make the stick work well with classic games.