Animal Crossing 3DS will add swimming, more home customisation and the chance to dress up in Legend of Zelda garb. It'll almost certainly make use of the 3D screen in clever ways and will feature StreetPass and SpotPass functionality in some form.
Animal Crossing on Nintendo 3DS has yet to be given a release date, but a 2012 launch is likely.
More 3DS coverage from E3 2011:
> Super Mario 3D: Preview
> Super Mario 3DS trailer released, out 2011
> Luigi's Mansion 2, Mario Kart revealed for 3DS
> Kid Icarus, Star Fox 3D out this year
> Zelda: Four Swords free for DSi users
> Zelda: Link's Awakening available from 3DS store
View Animal Crossing E3 2011 screenshots and a trailer below: