Quad-core phones. The only thing I'm certain that they'll do is extend your battery life.
I could go off on a tangent and say things like they'll allow your phone to go faster and the more cores you have the less strain so blah blah, but I'm really not so sure about those things. Just a small reflection here but slightly more than two years ago I bought my OG Droid, clocked at 550MHz with 256MB of RAM. Now, two years later, we have phones with 720p HD screens, 4G LTE technology, dual-core 1.2GHz processors, and 1GB of RAM. In two years, the specs of the most high-tech phones more than doubled. And I find that to be fascinating.