It's just that it was, for me, the first SW game in which I really feel to be IN the SW universe. The story was really good, the main character was absolutely cool,... and it was one of the first "intelligent" Doom-like at that time. I mean, the goal of each mission was not to find the blue key to open the blue door, find the green key to open the green door, then find the red key to open the red door... as it was in many FPS of these years.
I have really loved to play Rebel Assault II too. It was not a good game (I mean, there are too many different sorts of games in it, and the controls were really bad for some missions), but it was a great SW interactive movie, and a good experience for a fan.