Club Jade has a collection of coverage from today's Star Wars Rebels panel at WonderCon. The page also includes a video featuring Hera and Chopper in the Ghost cockpit facing off against TIE Fighters. The character animation is definitely more broad and, well, animated than TCW. It'll take some getting used to, but I like it so far.
They announced that Kevin Kiner will return to score Rebels, which is great news. They played the main theme, which evidently incorporates music from the films. The video has him playing a piece of music that may or may not be the main theme, but it sounds great!
EDIT: Rebels Report recorded the main theme at the presentation and posted it online. It incorporates the Main Theme and Force theme from the films, and I like it a lot – it's very evocative of the original trilogy. It's around a minute long, so I wonder how much of it will be played in the actual show intro?