I use my iPhone a lot to listen to music and podcasts in my car -- I connect it right up to my car stereo, and it's my main listening source while driving around LA. As a result, I really wish there were more options to control my music while driving around. Music- and podcast-streaming app Stitcher has introduced a new feature along these lines they're calling Car Mode, which is basically just a simplified interface with large, clear controls, so you can choose and play your music easily. There are also some other new features added into the app, including a new front page and faster playback. You can download Stitcher Radio for free on the App Store.
Unfortunately, this isn't really what I'm looking for -- this layout still requires you to pick up your phone and press buttons to use it, and that means that you'll still have to pull over to the side of the road to choose your music. (Because as we all know, using your cell phone while driving is illegal; just ask the LAPD.) What I'd really like to see from these streaming apps is Siri integration. I often will be driving around and get an urge to listen to a specific song or artist, and it'd be nice to use Siri to say something like, "Siri, play me some Rolling Stones," or "play me 'Bohemian Rhapsody,'" and have it automatically start playing.
I'm not sure that's entirely possible just yet, as Apple's API for Siri can be limited for some developers -- at this point, I think about all you can do is open an app. But hopefully that will change soon. The first streaming-audio app to let me start up and control songs in the car with just my voice will definitely have me interested in switching over to it.