The Reading List feature Apple introduced in iOS 5 gets an upgrade here. No longer just a way of shuttling links back and forth, it now also caches the links you add to it for offline reading. So if you are reading, for example, a very long review of a mobile operating system platform, you can add it to your Reading List and still access it while you're on a plane, the subway, or inside your lead-lined panic room. This works for any item in your Reading List, regardless of read/unread status. However, if my Reading List is any indication, it's worth noting that publishers can opt not to make their content available for offline reading.
It's a good thing Apple added that feature to Reading List, because otherwise iCloud Tabs might have rendered it largely obsolete. One of my favorite new features of iOS 6, iCloud Tabs lets you bring up a list of every open tab in Safari on any iOS device or Mac that's logged into your iCloud account. So if you realize that you left a crucial page open on your browser at home--directions to your cousin's new house, for example--you can pull it up in a few seconds.
While it's easily accessible in Safari on the Mac and iPad, it's squirreled away a little bit in the iPhone's browser: You need to tap the bookmarks icon in the tool bar and find it in the top-level bookmark list. The list that shows up is divided up by device, and tapping any bookmark will load it in the frontmost browser tab.
One of the best features of Safari in iOS 6 and it's buried miles deep: Settings -> Safari -> Advanced. Flip on Web Inspector and connect your iPad or iPhone to your Mac and you can use the desktop browser's Web Inspector to tweak settings on the Web page you're looking at on your iPad. As you change elements using the Web Inspector on the desktop, those changes will be reflected immediately on your mobile device. It's impressive to see in action, and it'll likely be a boon to Web developers who fret over compatibility with the latest devices.
In addition, just as you can tap and hold on the new message icon in Mail, if you tap and hold on the back or forward button in Safari, you get quick access to your browsing history for that tab (or your browsing "future").