Very poor battery life and the phone would always run hot. Swapped out the battery, volume buttons and the rear glass. Rear glass is just for looks. The battery life is so much better now, if I don't use the phone the battery keeps its charge...I can't even remember it being this good when I got it.
Super simple really. A monkey could do this.
If you are having problems with your volumes buttons, just disassemble them, clean off the dust and put it back together. Worked for me.
iPhone 4S Transparent Rear Panel
iPhone 4S Replacement Battery
iPhone 4 and 4S Volume Button