Sitting in the dark in the theatre, and having the ship come into frame from overhead and going "Wow!" and then a few seconds later, the Imperial warship powered overhead after it, taking up the whole sky.

That trick's been done since, but SW did it first, and the impact was just galvanizing. For all the whinging about how bad the FX in the first one look compared to the second trilogy, SW was state of the art at the time, and changed FX radically, forever after.