"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Side by Side Photos of iPad 2 and iPad 3 Casing Shows Gradual Taper and Slightly More Thickness
Seen on top of each other, the iPad 3 does appear to have a more gradual taper to the edges than the iPad 2. The difference in size, however, is said to be minimal but seems exaggerated in this shot above due to the staggered positioning of the casings. The next shot shows that the difference in thickness is only 1-1.5mm, with the iPad 3 enclosure being slightly thicker than the iPad 2.
MICGadget also confirms that the iPad 3 camera lens hole is "noticeably larger", suggesting that there has been some improvement in the camera quality, though they seem to question whether or not it will make it all the way to 8 Megapixels as claimed by one publication.
These same iPad 3 enclosures have been seen in the hands of many sites now. It's not uncommon for parts and enclosures to leak out of Apple's supply chain as production of new products ramp up. The iPad 3 is believed to be in manufacturing at this time with an expected announcement date on March 7th.