The Android L Developer Preview is out and available for most Nexus devices, but it's a bit buggy. If you've been itching to try it out, you're probably also nervous about whether or not L is ready to serve as your daily driver. I'll tell you right now, it's definitely not. The preview is not even an alpha, let alone a beta software release. The firmware itself isn't ready to be used as your DD and there are still a ton of apps that haven't been updated to work properly with it. The process will take time. That doesn't mean you can't get the Android L look and feel on your current KitKat device, though.
This trick does require root and you'll need to be running Xposed Framework already. After that, the two apps you'll need are on the Play Store. First, download the Google Now Launcher. Install it and set it as default. Then head back into the Play Store for the second app you'll need. It's called Xposed GEL Settings. Once you've installed it, launch it and head into the Menu. Click on General and then find L Launcher Design. Select it to turn it on, and then check the "Don't show again" box. Select "Got it" to confirm, and you're all set. It's not perfect, but it's a taste of what Android L looks like, built right into your launcher.
Whether you're looking for Android L visuals or not, you should install Xposed and start playing with it. There are a ton of really cool options there. Another way to get your rooted KitKat device to look like Android L is to just install the Android L Theme module for Xposed. You can find that on XDA. The module will theme your whole device with L stylings, not just the launcher like Xposed GEL Settings. We're talking your status bar icons, Settings screen, boot animation, and more. Be careful, because tinkering with the Xposed Framework can mess up your device if something goes awry. Have fun!