Haven't received my ipad 3 yet, but from what i hear from colleagues and my ipad 1:
A6 28nm with lower thermal output.
128GB Space.
Stereo speakers that switches in landscape/portrait mode (4 speakers, 2 at a time).
GPS in wifi models.
Reverse the volume buttons in landscape. (so the volmue bar grows in the direction of button push).
Better response times on the touch grid (think is 100ms right now).
siri.
Access to movies and tv series worldwide at the same date, no "this is not available in your country ****".