StarCraft 2's upcoming expansion Heart of the Swarm will add a new progression system. Players will be able to earn XP and level up by playing matches on official Blizzard maps or modes.
Experience points will be doled out at the end of each match. The exact amount you get will be determined by your performance, so build a lot, farm up those minerals, and kill scads of enemies. You can also get bonus experience through special circumstances, such as winning your first match of the game or playing with a friend. Blizzard is considering bonus XP events as well.
Your experience level is separate for each of the game's playable races. You can level up to 20 with each. By progressing, you'll earn new portraits and decals for your profile. The rewards for each race's progression are different, so you've got three grinds ahead of you.
Do people really care about decals? Not really. But if you add some sort of quantifiable goal to a game, people are going to pursue it. This seems like a really simple way to keep people engaged in SC2.
The leveling system will be available once the next patch for Heart of Swarm's beta goes live. Blizzard will periodically reset the XP of testers as they try to tweak the system. They want to make sure that the pace of leveling and entertainment factor are both high.
Heart of the Swarm is expected to launch to the public early next year.