One thing I hate is when I say 'OK, Google' and my watch, phone and tablet all try and respond! I wish it was possible to customise the trigger phrase between devices in some standard way, e.g. 'OK, Watch', 'OK, Phone', or a shout-out to Radiohead with 'OK, Computer' and maybe even something like automatically recognising 'OK, ${HOSTNAME_OR_BLUETOOTH_IDENTIFIER}'...