Besides the great game play that made Starcraft great, there is also an equally excellent storyline that goes with it. This why Starcraft stands out so much from other strategy games. Anyone that played the original Starcraft probably still remembers the betrayal and Kerrigan's sad fate.
The ability to tell a story within a grand stage relies on the audiences attachment to the characters. Stars Wars in the Phantom Menace and the following movies forgot this and focused more on the overall picture (The Senate, Politics, etc..), losing the audience that was unable to attach itself to any particular character. Avatar succeeded because it focused on just a few characters and told the story from that point of view, easing the audience to the themes and major conflicts in the movie. Starcraft does the same and to me the focus has always been Mengsk and Kerrigan, just like in the trailer above.
Here is a small recap of the story, to jog the memories of the people that have played this before and a nice primer for new people with interest in the game. It doesn't tell the whole story from the first game, but you'll be able to understand the Ghost of the Past trailer easier with this: