Through the new system, you’ll “earn XP for the race you’re playing with each unit you build or destroy”. So playing the game basically. Each race will have a maximum of 20 levels, with goodies unlocked along the way. What sort of goodies? Well, portraits for your profile, which Blizzard promises will “get more and more epic as you progress in a race”.
I don’t think I can muster as much enthusiasm as they have on this, but it seems to be the beginning of a new meta-game for Starcraft II.