Characters actually grow and change realistically and has reoccurring villains that have real depth and you never know quite what they are going to do. kai Winn and gul dukat both grow and change from series beginning to end and are the best villains ever.
This show has depth. It not all simple good guy and bad guy. There are complex motives behind many of the bad guys. Even the federation does some very bad things when they are in dire situations. People really have to live with the choices they have made. Problems from one episode don't just end with the episode like with TNG or the OS. you have to watch this from episode 1 to the end in order.
This show is still very relevant today. Terrorism, genocide, martial law are all seen in this series.