Battery drain from background processes
Under iOS7, my 5s would normally run an entire work day and drop from 100% to 50-70%. When I first updated the phone to iOS8, I found it was dropping from 100% battery to about 60% battery in just 2-3 hours, and in the red by lunch. After a little playing I found two issues that were draining my battery:
1) Quite a few apps were using location services a lot cause a big drain. I turned off those apps' access to location services in settings. I may need a couple at times but I can enable them then disable them when done.
2) A couple apps appear to be using a lot of background battery that they didn't used to, so I now use them then kill them.
My guess is the OS made one or more changes that the apps haven't been optimized for and either an update to the OS will be forthcoming to make it behave more like iOS7 used to in these areas, or the apps will soon be updated to make them behave properly.