I really liked Kes. I thought that she was a good, if somewhat underdeveloped, character who could have been put to better use if the writers had come up with a plan for her. The Kes/Doctor relationship is possibly my favorite from the entire series. I think that there was a ton of romantic potential for them down the road had Kes remained on the series. The very concept of a character with a seven-year life span comes with a lot of built-in story potential but the writers didn't really take advantage of it. By the final season, Kes would be an old woman and there would've been great opportunity to deal with her mortality (perhaps Kes dies just as Voyager returns to earth... something very bittersweet like that to add a little emotional punch). I also agree with others who think that Kes and Seven of Nine could've had an interesting relationship.
I do think that Seven was an interesting character, and I really don't have an issue with her joining the cast. However, I think that there would've been room for Jennifer Lien to remain on the series, as well. I personally think that Neelix had outlives his usefulness as a character by season 4 and could've been written out, but he was their token weird alien (he had the most makeup prosthetics) and they probably wanted to keep him around for that reason alone. I really think that the best option would have been to just bump someone to recurring - either Garrett Wang, Ethan Phillips or Jennifer Lien. Use them maybe about a dozen times a season. Their names are out of the opening credits and they aren't as expensive (since they aren't a series regular) but they still get to be a part of the series and contribute.