Masks
The use of masks has been greatly expanded over Ocarina of Time. Link will be able to use the power of certain masks to change his appearance and give him special abilities. For example, one mask turns him into a Zora, enabling him to swim underwater. Another mask will turn him into a Goron, allowing Link to curl up into a ball and roll around. Yet another mask will turn Link into one of those annoying Deku Nut guys and allow him to fly with the use of a flower, in a hang glider-like fashion.

Weapons
Many of the classic Ocarina of Time weapons and items are back, including the hammer, the bow and arrow, hookshot, the boomerang, the ocarina, the bombs and the dagger, only now young Link can use all of them. There may also be some new items in the game that Nintendo haven't unveiled yet.

Real real-time
The game will progress in real-time. There's now a meter at the bottom/middle of the screen that shows the position of the sun or moon at all times. It's believed that the time which passes during gameplay affects the outcome and story of the game much more than in Ocarina of Time. This feature is no doubt a remnant from the game's planned 64DD incarnation, where everything you do in the game causes lasting repercussions.

New Bosses
Gaiden features completely new, big bosses. While the specifics of all the bosses in the game are still being kept secret, we do know of at least three: a Gohma-like creature, an oversized frog and a giant warrior.

Expansion Pak
Since the game was originally designed to run on 64DD, a unit that required the 4Mb RAM expansion, it's no surprise that Gaiden (now on cart) will support the RAM Pak. However, it's very likely that the RAM Expansion will actually be required to play Zelda: Gaiden, just like with Donkey Kong 64.

Gaiden is expected to ship March 2000 in Japan.

Courtesy of IGN.com