What always annoyed me was that it was it was always women that left, and always for behind the scenes reasons. It was NEVER about what was better for the series.
To be fair, sometimes those behind the scenes reasons dictate things. Farrell leaving? Can't be avoided short of recasting Dax, which would create it's own uproar. Though I guess they did pretty much that in their own Trill way.
I don't mind the fact they killed off Jadzia, though doing it to establish how evil Dukat is, was a bit redundant by that point. I always found it odd that the fire thingy was right where the symbionant was but somehow it was it that survived and not Jadzia. DS9 at least made is a plot relevant death with lasting consequences.
Yar's departure... again she wanted to leave. Not much to be done about that if she doesn't want to stay. Sure it was unfulfilling how she died, but that was season 1 tng at it's status quo. I always found it silly on her part she got out as quick as she could, then spent the next six plus years trying to get back into the franchise.
Kes is a bit more controversial and this is one that irritated me some. She was literally the one character on Voyager with the most interesting potential but the writers just couldn't figure out what to do with her. Still think they'd been better off getting rid of Kim, but some fangirl at TV guide decided he was hot so he made that list.