How do I re enable a permanently disabled Notification Center in Mavericks?
If you ran sudo defaults write /System/Library/LaunchAgents/com.apple.notificationcenterui KeepAlive -bool False, it actually disables Notification Center because it changes the permissions of the plist from 644 to 600. If you change KeepAlive to false some other way that doesn't change the permissions of the plist, the NotificationCenter process is still started at login but it is not relaunched if it terminates for some reason. To undo the changes, run f=/System/Library/LaunchAgents/com.apple.notificationcenterui.plist;sudo defaults write $f KeepAlive -bool true;sudo chmod 644 $f;launchctl load $f.

If you ran launchctl unload -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist, run launchctl load -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist. launchctl unload -w modifies /var/db/launchd.db/com.apple.launchd.peruser.$UID/overrides.plist and sudo launchctl load -w modifies /var/db/launchd.db/overrides.plist, so you could also edit those files directly.