I actually preferred the original Jadzia. "Jadzia 2.0" as she was called in a previous post was too loud, aggressive, and wild. She was like a college sorority girl. Jadzia 1.0 was more subdued, more level-headed, and routinely coy, letting others chase her while not giving anything away. 1.0 was more Yin/subtle while 2.0 was more Yang/overt or loud. The character change didn't really bring anything to the character, it actually took something away.
Now, the behind the scenes reason is they wanted to tack her onto Worf, but I think the unspoken in-story reason was Curzon took her over. Look at Jadzia 2.0 then look at Curzon in Odo in "Facets". Notice how Jadzia 2.0 seems more like Curzon than like Jadzia 1.0. Curzon definately had a dominant personality and it seems once there was a constant Klingon around, his memories of wild romps with the Klingons sort of took over and overwhelmed Jadzia 1.0.
As for Jadzia's character development being kind of bland, I think there are a few reasons for that. They tried to explore Trills a little early, with "Dax" and "Invasive Procedures", but the 1 season they really hit a home run was Season 3, with "Equilibrium" and "Facets", but beyond that (besides "Rejoined"), what Trill/Dax episodes were there in Season 4-6? The other reason is her role. She was a science officer*. Early on, there was use for her, with the crappy generic TNG scripts of S1 (and a few strays like "Rivals") and then the exploration of the Gamma Quadrant (most prominent in S2, but also beyond). By the time the war starts, science is on the backburner and investigation isn't a priority, but... how much investigation did they use her for in Season 5? Last I remember is "The Quickening".
*: It was nice over all the series that they varied the gender up, that there was at least 1 female in each role in one of the series. Engineering... Torres, Chief Medical Officer... Crusher, Captain... Janeway, First Officer... Kira, peripheral fraternizing character (Quark, Neelix)... Guinan, Security Chief... Yar (yeah, not around for long). The only role that seems to have been missed is Ops (Kim, Data, Sulu or Chekhov).
As for Kira, she was abrasive but I never thought her a bad actress (nor an excellent one. She gets the job done, which is fine). I think some people were turned off by her early on by looking and acting like some weird hardcore militant (which she was). With intentionally unlikable characters, it's a balance between what their characteristics bring to the show and being an experiential eyesore/earsore that lowers people's interest in the show. Didn't some people way back in 1993 or so not like Quark?
That said, I don't think Kira was a big factor driving people away in Season 1. I think it was the utter lack of exploration, just leaving the wormhole, the initial draw of the show, idle (no missions to the Gamma Quadrant like in Seasons 2-4. "Captive Pursuits" & "Vortex", "Battle Lines" weren't enough. In fact, CP seems more like a Season 2/3 ep than a Season 1 ep) nor of the space on this far frontier of the Federation and beyond (like Ferenginar, Trill Prime, Cardassia, "Armageddon Game"). It got lame TNG scripts and focused too heavily on the Bajorans (it might of helped if there was a 2nd Bajoran on the major/supporting staff, like a Bajoran nurse that was more than an extra) and lame stories ("The Storyeteller", "Progress", "Past Prologue"). If it had stuff like the 3-part Circle story, "Necessary Evil", "Shakaar", it might have helped sell the Bajorans. Oh, and it didn't have the Cardassians as an active presence like they were in Season 2 onward.
Sisko seems to affect people like Kirk does with the acting style prone to nitpicking or grating someone at some point. But yeah, the producers allowed Sisko to become A Man Called Hawk. Of course, Janeway grates on some people too. Seems only Picard and Archer escaped being grating in some way.