I sort of think of DS9 as two different series, divided somewhere around Season 4 -- not to the magnitude of the division between seasons 2 and 3 of TNG, but still a significant shift.
Yeah, my mindset kind of goes along with this. Except for that bit about TNG -- IMO the shift in tone in DS9 season four is
much more significant than what happened between TNG seasons 2 and 3.
Even back in the day, I've always subconsiously seperated DS9 into two distinct phases: Seasons 1-3 have an all-round different feel, partially because there's that sense of DS9's isolation. One gets the feeling in the first three seasons that there are big events happening somewhere else in the Federation which DS9's characters only occasionally crossover into. Season 4, on the other hand, sees things heat up, and Deep Space Nine itself becoming ever more increasingly significant in Federation affairs.
I liken it to the station being almost a far flung outpost in the first three seasons, but being
the front line in the remaining four.
The point where it changes happens to coincide with a new title sequence, showing the station more bustling with traffic. It's a symbolic recognition that Deep Space Nine is suddenly THE place to be.
Whatever the case, it does make for a quite significant change in the overall way the show approached material. The addition of Worf, and Sisko's promotion to Captain and change of hairstyle, are also symbolic indicators of the show entering a new phase. Sure, the Jem Hadar and the USS Defiant had already been introduced previously. But in my own mind, "The Way Of The Warrior" feels almost like a soft reboot of the series' entire format.