I encourage you to watch this twice to absorb everything:
Death Watch, Onderon, a new female character named Stila (Steela?), Bo-Katan, Maul and Savage partnering with the Death Watch, a Republic Commando, and Palpatine appearing as Sidious and wielding two lightsabers against Maul and Savage. Wow. I got to see this trailer at the end of the season five premiere at Star Wars Celebration VI, and I loved that the crowd flipped out equally over the Republic Commando and seeing Darth Sidious.
I also attended the panel for The Clone Wars on Saturday morning. We saw a few clips of what's to come, and there's a lot to be excited about. TheForce.net has a detailed recap of the panel. I live-tweeted some highlights, so without further adieu (just remember to read from the bottom up on each image to get the tweets in order):