It's hard to keep a single storyline playing out for a long period of time. DS9 did it for most of its run, from the time we first heard of the Dominion until the very last show. They kept it going by tying it in with the newer stories--the war with the Klingons, the Dominion alliance with the Cardassians, for examples. In doing so, the writers kept the Dominion thread fresh and growing. It made for a lot of hours of outstanding story.
6. Taking chances with the characters.
Whether is be making Odo betray his friends, or Sisko conspiring to trick the Romulans into the war, or having Bashir turn out to be a genetically-enhance liar, the powers that be did not shy from taking bold actions with their characters. Anything a writer does that threatens to make his characters less likable is dangerous and subject to possible fan backlash. The writers took such risks often. And even if the follow-up went astray, the initial impact of tarnishing the shining armor of our heroes was not diminished.
7. Losing DS9 at the end of season five.
CRACK was the sound of millions of jaws hitting the floor as Captain Sisko packed up his Starfleet officers and abandoned Deep Space Nine to the enemy in "A Call to Arms." Sure Sisko had good reasons for giving the station up to the Dominion, but it didn't lessen the shock to those viewers who'd avoided all the spoilers and had no idea what was coming. It's always great when writers can still surprise their audience, it's even better when they can still pull it off five years into a show.
8. Introducing Section 31.
The Federation ain't perfect. We already knew that, but we didn't know how un-perfect it could be until Bashir met up with a sneaky guy named Sloan. Less noble than regular Starfleet, yet armed with essentially the same mandate of keeping the Federation safe, Section 31 shook things up in the DS9 universe. They breathed new life into the stories as well as the characters. My only complaint regarding Section 31 is that they weren't introduced earlier on in the series.
9. Allowing the characters to grow.
None of the characters introduced in "Emissary" were the same characters last seen in "What You Leave Behind." There was no massive cast upheaval along the way in which all the original characters were replaced with new ones. The main characters simply evolved. Through seven years they went through emotional growth and change--some
