Time's editor Rick Stengel said the Protester was chosen, “For capturing and highlighting a global sense of restless promise, for upending governments and conventional wisdom, for combining the oldest of techniques with the newest of technologies to shine a light on human dignity and, finally, for steering the planet on a more democratic though sometimes more dangerous path for the 21st century." Very lofty.
The initial gut reaction on Twitter seems to be one of derision, as Time has gone with a faceless human mass instead of picking a single person (like Tunisian fruit vendor Mohamed Bouazizi who Time mentions in the story and is widely acknowledged as the person who set off the "Arab Spring.") In 2006, Time chose "You" (with a mirrored cover) to much disappointment. They also once picked the personal computer as "Machine of the Year" and Earth as "Planet of the Year," proving that it should probably just be "Story of the Year" if they aren't going to acknowledge an actual person. By not picking any one individual, they've basically chosen no one.
Here's the full cover, which was designed by Shepard Fairey: