Game Of Thrones Season 7 Premiere: 9 Things You Might've Missed In 'Dragonstone'
Honourable Mention: The Hound's Cut Line
This one gets an honourable mention as, given it didn't actually make it into the episode, it wasn't even there for viewers to miss. In their recap of the episode, HBO added an additional line of dialogue to the Hound's scene where he looks into the flames (the key new bit is in bold):
"It’s where the Wall meets the Sea. The waves are frozen. There’s a castle there."
Given it was ultimately cut from the episode, maybe it's not worth reading too much into that line. But it does suggest a possible means for the White Walkers to pass into Westeros, freezing the sea to create a route in for themselves and their army of wights. Of course, the Wall is bound to come down too, allowing them passage, but having the sea would help given the scale of their army.