This is a complex question, with way more than one correct answer.
As has already been mentioned, it was made in the aftermath of 9/11, and in particular the American reaction to 9/11. It is hard to not see almost all of it as an allegorical tale of the Bush years.
It was not afraid of very difficult subjects.
It is hard to think of another TV show that made a sympathetic argument for suicide bombers.
It dealt with military overreach, torture, and war crimes; while still acknowledging the need, and respect for the military and the people who embraced it as a way of life.
It dealt with genocide, compromise with the enemy, religion, the abuse of religion, suicide, drug and alcohol abuse, and fate.
Workers rights, the rights of succession, and political system were all dealt with in depth with an intelligence rarely seen on television and never seen on a science fiction show.
On the production side it embraced a much grittier type of science fiction. There were no lasers, aliens, or “heroes.” The characters were all flawed, they committed crimes, they were selfish, and could be wrong for the right reasons and right for the wrong ones.
BSG was also a flagship show for a very poorly thought of network – Sci-Fi as it was then called. It brought the network a lot of credibility in the shape of reviews and awards. People who would have never watched the Sci-Fi Channel tuned in every week. There probably would not have been a Walking Dead, or even the current form of AMC if it were not for Battlestar Galactica.
In addition, and remember this is before twitter and Facebook became mainstream, the show makers – in particular Ronald D. Moore – embraced communicating with their audience. Ron Moore posted a Podcast commentary for all the episodes of the latter seasons. They also developed two webisode series which lead directly to the Hollywood Writers strike which almost torpedoed the final season of BSG. They were also okay with poking fun at themselves – the “What the Frak” series recaps pointed out the flaws in the show but it did not matter.
Finally, the show was just well made. It was very well written and directed. Many of the writers finished on BSG and moved on to Game of Thrones (it is hard to imagine GOT getting green lit without BSG). When Lost had literally lost its way, BSG had the same writers room freedom but was coherent and cohesive.
Battlestar Galactica ignored the usual tropes and talked about what mattered to people at the time. It showed what television could be.
(This originally appeared on Quora as an answer to: What made Battlestar Galactica such an important television series?)