This is great news. I will definitely be watching. This renewal has to be based mostly on the basis of the show's quality.
It's most likely based on the fact that AMC is desperate, Breaking Bad is ending, Mad Men won't be around much longer and they can't just rely on The Walking Dead. The Killing isn't great but they know what to expect ratings wise.
The only reason some people think of The Killing as a high quality is because of the first few episodes that were translations of the danish version, as soon as they started doing their own thing the quality went down the toilet.