simple. Go into recovery and make a nandriod, then..........oops. Can't do that when we aren't rooted. :d (this is my #1 reason to root - the ability to make nandroid backups.)
as i understand it, google can back up some of the stuff on your android phone to its cloud server (aside from gmail and contacts, which it does automatically) if you enable that when you set up the phone. I have not used it.
Esfile explorer can back up apps, but i don't know if you need to be rooted to use that feature. It does not specifically say the app backup feature requires root access in the description of it on the play store, so maybe it would work for you. What it does is put copies of all the apk files each of your apps needs to reinstall it over on your sd card.
Also, you may have some apps that are capable of making their own backups to the sd card (would be found in each app's settings) such as memento or mobisle notes, to name a couple i know of for sure.
Lastly, your cell carrier may offer such a service as well. I know verizon does, but again i have not used it so can't tell you how well it works.
Edit - one more thing you could do if you really want to stack your odds against any sort of catastrophe during the factory reset would be to power down the phone, pull your sd card, and copy its contents to your pc for safe keeping.
