I really wish there was something like f.lux (http://stereopsis.com/flux/) for iOS, or at least without having to jailbreak and do whatever else, because then I might be able to use my iPad in bed to read a bit before sleeping.
Having it on my laptop had made a big difference -- I "felt tired" when the display temperature was adjusted at nighttime. Before that, I could happily stare at that blue light and stay awake as long as I wanted, at the expense of my productivity the next day.
There's a lot of other tweaks which make iOS a much nicer place. More recently, Nitrous is a pretty amazing tweak which enables the Nitro JS engine for non-Safari apps. Speeds things up big time.
Aside from that you can have a way of having a particular browser open besides Safari, scrobble your iTunes tracks automatically, password protect apps, have a much better lockscreen experience, much better folder experience, theming, retina-fy low res apps, have a download manager, etc.
For me personally, there's plenty of reasons to jailbreak.
The current version of iOS is easily jailbroken and f.lux is a free package, available right from cydia without any detours. It's not as smooth as a built-in solution would've been, but much better than I ever expected.
Having it on my laptop had made a big difference -- I "felt tired" when the display temperature was adjusted at nighttime. Before that, I could happily stare at that blue light and stay awake as long as I wanted, at the expense of my productivity the next day.