When I upgraded to the iPhone 4 I found that the feature I found most useful was the LED flash. Not because it helped with pictures (I rarely use it for that), but because it made for a great flashlight. However, it does take a moment to get the app up for it, since you have to unlock the phone, then flip to the right screen and tap on the icon. I’d rather be able to press a button as soon as I pull out my phone and have it turn on. Well, if you have a jailbroken iPhone 4, there’s an app for that.
FlashCam is an app for your jailbroken phone that puts two extra buttons on your lock screen. One for taking photos, and the other for a flashlight. Honestly, I’d say that adding these two buttons goes at the top of my wants list for iOS 5.