The freedom to whatever I wanted, not what the game told me to do.
The sheer scale. I'd often read about the size of our galaxy, but Frontier was the first time I ever got to see a demonstration of just what that meant. It took hours to scroll across the map. Then there was the galactic map, which started off zoomed in where every pixel on the screen was a 8 x 8 light year sector, and then you could zoom out, and out and out... it was completely mind boggling, to be honest.
Being able to jump into a system and see the stars in the distance, and the distant specks that were its family of planets, and then locking onto one of those specks and hitting the star dreamer until the speck became a full sized planet with continents and oceans, and then descending to the surface, and landing in the middle of cities where it was possible to read the time on the clock towers, and watch strange stars and planets rising and setting in wierdly coloured skies.
Barrelling over the surface of a distant world at thousands of kilometres an hour at low level, flying by the seat of my pants with my hair on fire and my heart pounding, the Imperials on my tail and the range counting down to my objective. Zipping between the docking towers of the Imperial base, cameras clicking and then powering back into space atop a column of plasma, escaping into hyperspace as my shields fail and enemy lasers whittle away my hull.
The feeling of relief after being out in the wilds or on a dangerous mission, only to come back safely to a birth in a developed system for repairs and recuperation.