Nicolas Finney finds El Capitan has elided his ability to enter accents easily:
In iOS and Yosemite, there was a very convenient shortcut for inserting alternate characters (e.g., an ‘e’ with an accent ague (é)). The procedure was to hold down the base character (e) until a menu appeared so that you could select the alternate character you needed.
This remains available in El Capitan, but a system configuration file may have gone hinky. There’s a hidden that runs that manages this process. You can use Activity Monitor (in Applications > Utilities) to see if PressAndHold is running: Just enter that name in the monitor’s Search field and see if there’s a match.