At this point, a few months into things, I'm pretty big fan of Android Auto, warts and all. It's early for AA as a platform, but it's also what I think is going to be one of the most important improvements in the mobile space, if only because it'll help get phones out of drivers' hands. The necessity of that can't be understated.
That said, there's one glaring thing that I'd like to see fixed as soon as possible. And that's the app picker. Android Auto apps currently are split between messaging apps — which don't actually appear anywhere in the user interface until you receive a message — and media apps. Music, podcasts, news, etc. And if you have more than one media app installed (Google Play Music will already be there, of course), you switch by tapping the headphone icon in the nav bar a second time. (Or once, I suppose, if you're already in a media app.)
There's a lot of wasted space in the Android Auto app selector, and going 75 mph isn't the time to be scrolling through lists.
That pulls up a list of the media apps. And as you can see, there's a lot of wasted space there. If you have more than three media apps installed, you'll end up scrolling to get to anything other than the first three listed. And scrolling through lists is absolutely not something you want to (or should be) doing when you're controlling a couple thousand pounds of steel and plastic and people hurdling down the highway.
I'm generally not a big fan of Apple's "Springboard" user interface — grid after grid of app icon generally bores me to tears, though Apple's done a decent job with the UI in CarPlay — but in this case Google needs to rethink the card metaphor. The information density of this app picker has to be improved. Or we as users will need to limit how many things we have installed, and that's definitely not the right way to go about things.