I like how people frequently point to Nintendo as the prime example for milking franchises for all they are worth as if other first-party companies don't do the exact same thing (Microsoft with Halo and GoW, Sony with God of War, Ratchet & Clank, and Jak & Daxter). In some cases, third-parties milk it for even more (see: Activision with Guitar Hero).
The example here, Mario Kart, doesn't even make sense as an example. The franchise only appeared once on each platform mentioned. Granted, each each iteration's differences were largely due to new hardware features. To recap, MK
D!! had LAN play, although the excellent "Co-Op in a Kart" innovation is truly hardware agnostic. MK
S had online play as a first. MK:Wii was a first for motion-controlled Mario Kart and provided an improved online experience. I know this isn't what's being said here, but to say that MK:3DS will only be notable for its 3D graphics is definitely premature at best.
To say that the only possible direction for games to be improved upon for the 3DS hardware platform is 3D visual enhancement ignores the array of other new features like the Augmented Reality/camera functionality, MotionPlus-level motion sensing (not implemented in MK Wii), StreetPass/SpotPass functions, and more things I'm sure I'm not coming up with right at the moment.
Some people are happy with waiting because they are not satisfied with the launch library. I understand. It's not to everyone's taste. But to say the 3DS is using technology dated to the point that there is no possible draw to owning the system if you can't have the 3D on is silliness.
And there's all sorts of flaws inherent in comparing it directly to a PSP, because the DS, which 3DS takes its form-factor from and is compatible with, was always considered drastically different from the PSP for its touch screen and overall game library.