As a die-hard Marvel fan, the Legion was one of the few DC properties that I thought was worth buying. Not living in the States, following any imported American comics in those days was virtually impossible unless you happened to live near one of the very few dedicated comic-books shops (I didn't), so my knowledge of Legion history was confined to a few issues here and there. And the Servant Shall be a Sign was the first issue I owned with Keith Giffen art, and Giffen's whole style looked futuristic, especially the way the colourist implemented certain parts of the drawing without a black-ink border. The whole mystery angle of the servants was interesting, especially as they hinted so strongly in the first issue that the first servant was a Superman clone. Seeing the various people and things that the master was collecting to boost his power was also fascinating, and gave me a glimpse or two into the Legion's history. Basically, everything about it captured my imagination. All the Legionaires got a chance to do things, too - no one member hogged the limelight, which I liked.
Overall it was an epic story in what was a top-quality run, with consistently good art to match the story. I still think it works well today, both as a read, and as a template for how to put together an interesting epic that doesn't screw any characters over in its attempts to generate drama.