Android should just blur the lockscreen wallpaper by default
I feel like one of the things I've learned from using Muzei is that blurred backgrounds are nice. In addition to looking good, they maintain a very clear visual distinction between between foreground and background elements. For example, here is my current lockscreen (Muzei + Androidwallpape.rs Extension):
The text is clearly legible, and the UI elements are obvious. Even if you've never used Android before, you should be able to intuitively tell the difference between things that you can tap or swipe (ie. the dots, the camera shortcut, the Google Now shortcut at the bottom) and things that are part of the background imagine.
However, if you play a media file, Android replaces the background with an image related to the whatever you're playing. This seemed like a cool feature when KitKat first came out, but it doesn't always look great in practice. For example:
Obviously, if you're familiar with Android, you know that's a swipe-to-unlock pattern. But it's not as intuitive as the first screen; it almost looks like the the dots could be part of the background. And note: this is a good background for showing off the foreground elements, because it's dark and relatively simple. If you have a media background that's mostly white, or full of detail, then it can be really hard to see the foreground controls. And even if you discount the issue of usability, I feel like having the same level of visual acuity for elements you can tap and elements you can't tap on is jarring and kind-of ugly.
Solution: Android should blur and darken media backgrounds by default, the same way that Muzei does. If you don't see the point of blurred album art, then take a look at the Rdio app, either on the Android or the web. It uses blurred album art for its "now playing" screen, and it looks really, really good. Android should do the same thing on its lockscreen.