It's the little things. Since the introduction of Time Machine in OS X Leopard, Apple's built-in backup utility has provided a safety net for millions of Mac users by delivering effective, dead-simple data protection. Every hour, the latest versions of our files are neatly copied to external drives or to our Time Capsules, and we are happy.
Some of us could be happier, however, if we had a no-hassle way to use Time Machine with rotated media. Taking a drive offsite while keeping one at home is a long-standing backup best practice, and swapping them every week or month is a great way to protect against local disasters (floods, fire, electrical overload) that take out both your computer and the nearby backup media. Time Machine has supported multiple disks previously, but you could only target one drive at a time; you had to manually reset TM to point to your drive of choice.
In Mountain Lion, that changes. Time Machine has gained full support for multiple listed target volumes; just add the additional backup drives in System Preferences. Time Machine will automatically rotate backup targets, using all drives alternately -- or if only one is available, it will target that until the other drives come back. Restores default to using the most recent available backup set, as they should.
Not everyone's been waiting for this particular Mountain Lion feature, but I sure have been.