The thing about Jadzia is, she was a great idea for a character, but how do you develop somebody who has lived eight lifetimes already?
I liked what they did with her in Dax, but there's only so many times you can draw from the metaphysical 'who is she really' well. They should have committed to making her a comedy-oriented character.
The writers never knew what to do with the Jadzia character. They should have just written her as an old mentor type of character from the start.
We also got Kes from a race that only lived a short 7 years, but the writers on THAT show didn't know what to do with that character either.