A long tradition of Nintendo backwards compatibility has been broken.
Nintendo have been the paragons of backwards compatibility and continuity through the generations- every single one of their handhelds has been compatible with the one that came before it (Gameboy Advance with Gameboy and Gameboy Color, Nintendo DS with Gameboy Advance, Nintendo 3DS with Nintendo DS and DSi), and their consoles from the Gamecube onwards have all been backwards compatible.
However, that ends with the Switch- the new system will be backwards compatible with neither the Wii U nor the 3DS, Nintendo confirmed in an interview with Japanese magazine Famitsu (translated by NeoGAF).
Speaking about the system’s battery life, Nintendo confirmed that they have nothing specific to say, but did drop the hint that “the development team aims at allowing players to play NS conveniently at places without electricity connection.” Which sounds encouraging, but is vague enough to be ominous. Hopefully Nintendo don’t screw up the battery life here, because that would significantly undermine the main appeal of the system.
Nintendo Switch will launch worldwide in March 2017.