I think the main thing is your screen. Your screen was on almost all the time. The big, bright screen on these smartphones suck battery like there's no tomorrow. There's no much you can do about that other than turn the brightness down or buy an extended battery (Or charge more often).
Also, understand what these percentages mean. All battery use adds up to 100%, so when other things aren't using battery then System and OS, by default, get assigned the usage. 15% isn't out of the ordinary.