Another difference is perhaps that Ivanova wasn't killed off, and you knew that "leaving the show" didn't have to mean "never to be seen again" (see: Sinclair). Whereas not only was Jadzia Dax killed off, her death was the worst kind of fridging, not about Jadzia at all, but about providing angst to her menfolk and showing that Dukat is really really evil now (tm), what with him being in the s6 finale for the sole purpose of lurking in the Bajoran temple and shooting Jadzia (who never went there before) because hey, why not. A death like this is found to influence how one feels about the successor even though it's not the next character's fault.

Then again, I've seen people refuse to watch s5 for the sole reason there's no Ivanova in it (save for the series finale). So who can say how that works on the fannish heart...