How do I re-enable Notification Center in Yosemite or El Capitan post-upgrade?
This in Terminal will show Notification Center again:

launchctl load -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist
killall NotificationCenter


You will have to restart your Mac afterwards.

Reported to work in El Capitan (10.11.1) as well.