This is one of the hottest and most controversial topics on some trek related bulletin boards.
But here is the story, heck I have heard it so many times, I can say it by heart.
Originally Garrett Wang (Harry Kim) was to be fired from Voyager.
All cast members original contracts (with the exception of Kate Mulgrew I think) expired at the end of season 3, and where being renegotiated.
Originally the cliffhanger to season 3 was not suppose to involve the Borg or Species 8472, but actually meant to be "Year of Hell, Part 1", already forshadowed in the Kes episode "Before and After". Also it is to note that "Year of Hell" orginally didn't involve time ships, reset buttons or any of that jazz, it was a medium sized arc for season 4 showing what happens when Voyager gets into bad trouble.
But at the time it was decided that some new fresh blood needed to be introduced into the show (to help boost the ratings), and thus the producers decided on creating the character of a Human, Aassimilated into the Borg at a young age and then forced by Janeway to be reintroduced back into human society. So the entire Scorpian plot was developed from there.
Now the sticking point was that the producers wanted to keep the cast at 9 members, not have it go up to 10, so somebody had to go. And originally it was to be Garrett Wang.
The Producers didn't particularly like Wang and he was known to be lazy (he was suspended from 2 episodes late in season 3 - "Blood Fever" and another one due to his constant lateness to the set every day).
At the end of Scorpian Part 1, Harry Kim, after being striked by Spieces 8472 was dying - thus killing off the character.
But something happened during the summer hiatus that changed everything - Garrett Wang was choosen as one of People Magazine's 50 most beautiful people for 1997. The producers, for goodness knows what reason saw this as a sign and decided to keep Wang. But at this time Jeri Ryan had already been signed onto the cast and the producers need to get rid of one of the cast members. The axe unfortunately fell on the lovely neck of Jennifer Lien.
Some people accuse Jeri Taylor of delivering the pink slip, though I for one do not know.
So the episode "The Gift" was written at the last minute as a send off for Kes.
Personally I am disapointed Kim stayed and Kes left. Kim spent the next four years hanging around like a wet rag, and the few decent episodes he had (Timeless etc) while good, could of been done by any character. (Actually it is my opinion that Paris would of been more suited for Timeless then Kim).
On the other hand Kes had come into her own during season 3 with fantastic Kes episodes like "Warlord" and "Before and After" and a great b-plot in "Darkling" plus she already had storylines in the work for her for season 4 (the retouched version of "Year of Hell" is just one).
She was also one of the most unique characters in trekdom, and it was a shame she never got the chance to proberly develop.
And just a note on Jennifer Lien's slight weight gain in "Fury". Jennifer Lien had by that stage, dropped out of the Showbiz scene and was starting a college course, thus she had no real need to stay thin and went to a dress size that she was comfortable with. Heck - gaining weight is something many of us do in college! Including myself!
All drug addiction rumours that have circulated that internet about Jennifer Lien are totally false and incredibly nasty.
So that is the story...