With S-Off, you can flash kernels in recovery (without having to connect to your PC and use fastboot), you can completely return to stock without having the words "tampered" and "re-locked" glaring accusingly at you from your bootloader screen, and you can flash new radios and change the splash screen on your device, just to name a few. These are things that many of us in the Nexus and Samsung worlds take for granted, but they take much more effort from the development community for HTC devices.
Dubbed "Facepalm," the new S-Off method was developed by XDA/RootzWiki members beaups, Dr_Drache, dsb9938, and jcase and works on the Snapdragon S4-based One XL, One S and Droid DNA. Before going any further, I wish to stress that DNA owners who want to gain S-Off access on their devices right now should NOT apply the latest OTA from Verizon unless you have already achieved SuperCid bootloader unlock on your device. If you haven't done this, and you have already accepted the OTA, you will have to wait for an updated method.
The instructions are pretty straightforward and can be found here, but you need to know what you are doing. You must have the Android SDK installed on your PC and a basic knowledge of adb and fastboot commands. If you don't you probably shouldn't risk trying to get S-Off on your phone. Neither those who developed the method nor RootzWiki can be held responsible for any damage done to your device - you've been warned.
One more note: to answer one of the most common questions posted in the HTC forums for new devices, S-Off is NOT part of the process to get AOSP-based ROMs working on a Sense-based device. AOSP depends on having a working RIL (radio interface layer, which enables phone calls, WiFi and Internet access) independent of Sense. With a working RIL, AOSP could be ported to an HTC device even without S-Off.
You can see the S-Off method demonstrated by jcase in the video below: