has a good article on troubleshooting 8 known issues with iOS 8. The issues they cover are

  • Wi-Fi connectivity issues
  • Battery drain
  • Safari slow page loads
  • Low volume through earpiece during phone calls
  • iMessage errors/can’t connect to server
  • Apps not updating/stuck downloading
  • Slowdowns/sluggish behavior
  • No phone calls in or out

If you are having any of those issues, you can check out their possible solutions.