I like both the story elements, and the melee. But I also like how I can move about freely in the game, rather than being required to do everything in a specific order. It being set in space, with interstellar travel is a big bonus as well. SC2/UQM is my 2nd favorite video game of all time.
Definatly the melee.... in fact, i havent played the story for more than 5 minutes....
I originally got hooked from SC1 which had only melee. I played SC2 Melee on my friends 3DO a bit, and hadent heard of it for a long time, until UQM. These days I have so many games to play that I dont have time for long games anymore.... but someday I will.....someday.