Man, the Protoss were the ones I usually played when I played the first StarCraft
game online. While the first of three coming from Blizzard will focus on the Terrans, Star Craft II : Wings of Liberty will also let you control
a little bit of the Protoss as well.
The mini-campaign is designed to give you a taste as well as be used to further the story along in the single player portion of the game. Now, you won't control all of the Protoss units but you'll definitely get a handle of how some of them will be in the game. The small taste won't even get you ready for multiplayer play with them but it will add a little variety as you won't just be playing the Terrans when the game's released.