The SNES was a giant leap over the NES in audio and graphical technology; it had DKC, which challenges titles even now with its beautiful 24-bit rendered graphics - made possible by the SNES graphical chipset, which normally would allow a 32,000 colour palette. Why did it sell less than NES? Sega Genesis dominated for too long in the generation, it took until 1994 for the SNES to really become the IT system (with games like DKC and FF6), and then Nintendo killed it early in 1996 when N64 released.
N64 was the first fully 3D console, it also was the first Nintendo console which took advantage of expansion (RAM, Rumble, Memory card), and had analog control which it popularized. The problem with N64 was that PSX was WAY cheaper, the games were about half the price, and there were WAY more of them - also Square supported PSX exclusively. The N64 had 4 games for its first 6 months while the PSX was advertising that it had over 200 games. Also, NO RENDERED 24-bit GRAPHICS, that is the technique that SAVED the SNES from Sega dominance.
The Gamecube was supposed to be the first motion control system (Nintendo had the patents during the N64 era); everyone competing was worried about the rumoured crazy new Nintendo control - this just didn't come until the Wii. The Gamecube was Nintendo's generation off.
Anyway, I can't see the PSP2 being as successful as the original PSP considering how many people felt burned by the low support for the system - the PSP2 is going into the race slower than the original PSP, the 3DS is going in much faster than the original DS. I think the 3DS will do better, and do it faster than DS did.