Nintendo Switch will not be capable of playing physical 3DS cartridges or Wii U discs. Japanese gaming site Famitsu reached out to the developer and received a few further details on Nintendo's new hardware. The following is a rough Google translation of Nintendo's answer on the matter...
"Since the Nintendo Switch is not a Wii U or Nintendo 3DS successor machine, you can not play with Wii U disk software and Nintendo 3DS card."
Of course, Nintendo will likely utilize their eShop to port some titles into the console so more than likely some of these titles will be available digitally.
One thing that was apparent was that Nintendo was eager to stress that this is a "brand new console", which sounds silly to state but after confusion on the Wii-U launch better safe than sorry. Personally, I would argue that the Wii U almost certainly laid the groundwork for the Nintendo Switch and that's apparent by the tablet and gamepad setup.
Other than that Nintendo didn't have much to offer and said more information will be announced at a later date.