Possible sick plays: Opponent pops standstill, respond with E.tutor for half of key/vault. Draw it in your three. EOT Snapcaster to replay e.Tutor with your 1WU open, draw step you get your other half and win. Of course Jace fateseal ruins your day.
This doesn't work. If your opponent plays a spell that triggers Standstill and then you play a spell in response, this will trigger Standstill again. Your opponent will get to draw the 3 cards. Standstill triggers on every spell that is played, and it doesn't leave play until a trigger resolves. This is why you can't Stifle it.