Deep Space Nine is a cult TV show within a cult TV show phenomenon. I'm told not all Star Trek fans love it, but the difference between the ones who do and the others who liked Voyager or Enterprise is that fans who love Deep Space Nine love it passionately. We're an entire subculture of the fanbase.
"The Nagus" is the real initiation of the show's breakout status as a mini-cult. You don't have to love Star Trek to love it. As the title implies, it's the introduction of Grand Nagus Zek, the Ferengi ruler who would make subsequent appearances in nearly every season, and thus a key component of the rich mythology that fans grew to appreciate. Although not as significant as Garak or Dukat (or ubiquitous as Morn), he's someone you love to see every times he stops by.
"The Nagus" is the second episode in a row to put the spotlight on Quark, predictably in a story about manipulation, a famous drama involving Zek's son, who never appears again. It's also a little disorienting to see Rom, Quark's brother, in this and other early appearances, since he hasn't yet maturated into his finished form (much like the rest of the series).
The other good news of the episode is that it's an early test of the friendship between Jake, Sisko's boy, and Nog, Rom's son, one of the defining relationships of the series. Maybe no one will ever mistake Deep Space Nine as the Jake & Nog show (or perhaps, consortium), but you'll be hard-pressed to find better examples of what the show was all about.
This is the first Ferengi episode, too. Even die hard fans of the series were hard-pressed to like these perennial efforts, but the true connoisseur knows them to be an essential ingredient.
franchise * series * essential * character
Memory Alpha summary.