The big news from the first day of the AAS can be summed up with "Kepler 10b", which is the first confirmed Kepler planet that's mostly made of rock. It's also the smallest exoplanet ever found at 4.6 earth masses. Talks today by many of the Kepler science team focused on this little ball of burning hot rock which orbits its parent star roughly every 12 hours. This suicidally close orbit leads to surface temperatures in the ballpark of 2500 K, which means it probably looks a lot more like a super-Mercury than a super-Earth. One scientist compared the newly discovered planet to Dante's Inferno. You can read the media articles here, here, and here.
From a planet formation standpoint this little guy is also a big deal because it is significantly more dense than Earth, Venus, or Mars, meaning that it is either 75% iron or it contains some material in its core that is so dense it doesn't exist in our solar system. Laboratory experiments are starting to show that at the ridiculously high pressures one might find in a planet like this one you can get some really funky phases of common materials - things like metallic hydrogen or super-dense ice at 2000 degrees. It turns out that rocky exoplanets might help us understand high-pressure physics in ways that are really tough to do here on earth.