While the preview images for Android's upcoming "L" release just came out yesterday, the new firmware has already been rooted, as is standard with its openness. So if you were feeling reluctant to try Android L because it didn't support existing root methods, then you might want to reconsider.
Update, November 12, 2014: Android 5.0 Lollipop is officially out now. If you haven't gotten the OTA yet, check out our new guide on installing the official Lollipop builds on any Nexus for download links and instructions, for Mac or Windows.
One of the best things about the Nexus line of devices is the ease with which one can achieve root. Plug your phone into your PC, download a couple files, type a few commands, and you can be rooted within 30 minutes. No muss, no fuss, no carrier-locked bootloaders.
Update: June 19, 2014 Android 4.4.4 has been released, fixing a small security bug in 4.4.3. The factory images have already been made available to all current-generation Nexus devices, and I've linked them out below. This does not affect the process depicted, you can still use these same steps to bring your device up to date with this latest release.
Okay, so you're rooted and need to return your Nexus 5 to its stock firmware. Whether it's because you need to send your phone in to the manufacturer to fix a defect or you just want to use the device's over-the-air updating feature, unrooting and returning your Nexus 5 back to its fresh-out-of-the-box state is a fairly simple process with the right tools.
When you enter the softModder realm by rooting your Nexus 5, you open up an exciting world of possibilities and deep customizations for your device. Among other things, it gives you the ability to flash custom ROMs which, individually, act as entirely different operating systems that can vastly alter your user experience.
The flexibility within Android is pretty amazing. Unlike iOS, you can make a lot of modifications with nothing more than a download from Google Play, but to really step up your softModder game, you'll need to go a little further.
A rooted Android device means we're on our way to some truly amazing customization options. But there may be a few steps left to completing that journey, and a major one is installing a custom recovery.
It didn't take long at all for developer Chainfire to sort out a root method for the new Android 5.0 Lollipop. After handling a few issues with changes to the SELinux kernel module, Chainfire pushed out an update to his popular SuperSU root app.
It's time. You've experimented with apps from the Play Store, you've switched your lock screen, and maybe you've taken different launchers out for a spin. If, throughout your tinkering, you've found yourself chanting, "More. More. Give me more!" then now is the time. It's time to root.
You know what's awesome? You own a Nexus. That means, with Google's blessing, you have easier access and control over your device than other manufacturers allow. You can really dig in and do some interesting things if you root.