Regarding Eko's departure, this is what Kristen from E! had to say about it:

From Jason: Why was Adewale Akinnuoye-Agbaje written off Lost? Was he as unruly as the rumors are saying? I don’t wanna believe it!
And as the creator of the moniker SexAAA, you know I don’t wanna either. But it’s now been confirmed to me by various sources that Adewale had some interpersonal issues on the set. Personally, I don’t want to soil my own karma by spilling it all (Mr. Hickey would be so proud), but let’s just say that when Eko appeared to be a gentle humanitarian, that was indeed an Emmy-worthy performance. (P.S., I also would like to retract the words "mean," "stinker" and "of a stupid head" from my former statement regarding JJ Abrams.) Meanwhile, a fellow E! Online reporter (Josh Grossberg) ran into Lost executive producer Bryan Burke this morning and asked whether Eko’s departure was something planned. Burke replied: "Story-wise, it was definitely the direction we're going. I don't know what our official answer is on all that." Burke later added that Adewale asked to leave the show and that "it was for personal reasons that we can't talk about."