Calista isn't leaving the show. She just asked for more time off this season.
I actually stopped watching a while ago, maybe around Thanksgiving. It's become more of the same and I'm just kind of over it. I watched sporadically last season and for the first part of this season and then I realized, after weeks of not watching, that I didn't really miss it. I used to enjoy the Fighting Walker Schtick, but now I'm bored with it. I like many of the actors on the show, but some of the story lines have gotten a bit preposterous. I hope they get back on track, because I used to really enjoy this show Then again, maybe it's just run its course.
I'm not positive, but I'm pretty sure Balthazar Getty was written off the show a while back because of all the drama over his affair with Sienna Miller. I read at least one interview with a fellow cast member who clearly was not happy about the fact that he was so openly cheating on his wife. They never said for sure that his behavior lead to his character being written out, but the timing was pretty convenient.