If there is one jailbreak tweak I particularly miss, it's Xpandr, a tweak that let's you create keyboard shortcuts, just like TextExpander does on your Mac. To me, it's an invaluable tool that has saved be hours of typing.
Keyboard shortcuts aren't a jailbreak-only feature though, as they were introduced with iOS 5 a few years ago. Though not as powerful as Xpandr, they're my fallback solution when there is no jailbreak available for my device.
So one of the first thing I did when I got my new iPhone last Friday was setting up a bunch of shortcuts (Apple calls them Text Replacement) that I use on a daily basis. Everything worked fine for a day, and then, they magically stopped working. Though I was worried I'd have to restore my iPhone to fix the issue, it turns out the solution was much simpler than I thought.