Well Skyler, Nintendo doesn't want Link to be a fully fleshed out character. Why? Because they want people of all ages to fill the void that is Link. If Link was given a specific personality, then it could (and would) alienate a certain demographic, be it the younger generation, the middle generation, or the older generation. Why the Zelda games continue to thrive so well is because Link is a void. The hero in green is an archetype for the player.
What they need to do is add the option to change Link's primary hand, gender (or maybe not because relationships with characters such as Zelda would change and dilute the story), and facial features so that it still feels like Link, but can be customized to the player's desire. That would further bond player and avatar to the world of Hyrule and give a great deal of replay value. Link does not drive the story, the other characters do. Voice acting wouldn't hurt the rest of the game, but leave Link alone, period. Bad voice acting greatly hurts a game, but on the rare occassion that good voice actors are used, it can certainly help a game. Dialog trees don't always work. While you're deciding what the player says, you're still limited and can't express your true feelings, thus creating a personality conflict between you and the protagonist. Even if you get to edit what they look like to your desire (as seen in Bioware's games), it doesn't save the fact that you can feel alienated by the limited choices in dialog. The fact that Link is all action and not dialog shows exactly what Nintendo cares about: gameplay. Don't fight it, accept it.
Also, a mass amount of dialog can ruin the mystery of the lore behind Zelda. That's part of the atmosphere in every game. There are little bits of unwritten history here and there throughout the series that is never fully explained through dialog because that would ruin it. Gamers would be stripped of their freedom to explore and analyze using their own intellect. On top of that, I'd feel robbed if they gave the characters in Zelda accents and voices that I didn't feel they deserved. If Link spoke and had a british accent, I'd cringe.
Actually, Link has spoken audibly in Wind Waker. He says, "come on!" to Medli and/or Makar in the Earth and Wind Temples.