Part of DS9's charm comes from the fact that there's character continuity. Unlike Voyager, or TNG, or even TOS, the "guest" characters don't simply leave at the end of the episode. People keep coming back to the station, not just passing through.
Enterprise looks to have the same advantage, since the distances travelled are so short ('cause of having such slow engines). Folks like Soval and Shran and (I'm hoping) Kolos can keep showing up.
The characters evolved and changed and showed more of their personal lives than the TNG or Voy crews.
It also benefitted from having RHW (Robert Hewitt Wolfe), one of the greatest SF writers on TV, involved with the last few seasons. Guy knows how to write arcs.
DS9 also has the (retroactive) advantage of a great finale, which ended the series arc, but kept the characters in circulation (unlike Voyager, which ended with the sound of a guillotine).
The novels, of course, always tend to be better than the TV series. Better-plotted, more epic, and with settings and events that just wouldn't be possible on a TV budget.
See, there're two main types of SF writers.
When we (including myself ^_^ ) start out, we're told by Everyone(tm) that it's Absolutely, Utterly Impossible to write real aliens. No matter how hard we try, the aliens will always end up as reflections of ourselves, as humans. 'Cause we just can't imagine anything really unusual or strange.
One type of writer accepts this, and goes on to write thinly-veiled social commentary.
The other (my kind --and RHW's, actually) tend to take that ex cathedra dictum as a personal challenge. We struggle to out-do each other with wierd, bizarre, and wonderful creatures, springing from the depths of a human imagination that is truly limitless.
Thing is, it's the conservative writers that usually end up writing for TV. It's just easier to meet the "lowest-common-denominator" demographic. The daring kind usually stick to text --like the Star Trek novels.
If you've watched Voyager, you've seen what the unimaginative writers did to the Borg. They just couldn't wrap their heads around the idea of a hive mind, so they turned the Collective into a collection of slaves controlled by a Queen.
Farscape is an example of what the more creative writers can do when turned loose. As is most of DS9.
"The Beasts know much that we do not." -Ancient Jedi proverb