You could see a few of them from the gameplay they showed. The Guardian outfit gives an attack boost (like the Barbarian armor) but offers no set bonus, the Midna Helm gives Guardian Resistance (like the Guardian armor set), and the Tingle outfit pieces don't have any bonuses, but grant Night Speed Up as a set bonus (similar to the Stealth and Dark Link outfits). Also Majora's Mask doesn't have any effects directly listed, but its description says that wearing it "makes it harder for certain enemies to spot you," which is pretty vague but we'll see when it comes out.
ooooh there's gameplay video? I assume that majora description means a slight stealth bonus. I wonder if people will freak out when you wear it like they do the dark link outfit.