Engadget managed to get a hands-on with Android L and already it's clear that there are some big changes on the horizon for your Nexus 5 and even Nexus 6 (if Google goes that route).
To be clear this is the pure Android experience, so for those that have phones which run overlays like Samsung's TouchWiz or HTC's Sense UI then you'll either need to do some tinkering or wait until Samsung and HTC forge their own new designs.
For now though if you've got a Nexus 5, Moto X, Moto G or Google Play version of the HTC One and S5 then you're in luck as somewhere down the line they'll almost certainly be on the list for the update.
Android L's designs are already starting to creep into the current version of Android. Google recently updated the Google Play store to include a newer, flatter image-heavy layout that will be the benchmark for the rest of the operating system.