Darn it, I liked Mr. Eko. He was one of my favorite characters. Michael Ausiello of tvguide has some insight on why his character was cut.

Ausiello: Another actor leaves Lost just months after he's pulled over in Hawaii for a traffic violation. This is the part where you guys tell me there's absolutely no connection.
Carlton Cuse: We actually work very closely with the Honolulu police department, and they have final script approval.
Damon Lindelof: Look, we feel very strongly about traffic violations, no matter how minor they are. All we can say is that Evangeline Lilly got a parking ticket last week, and she better not count her chickens.

It looks like seriously, the actor just didn't want to commit long-term and this was the best way to complete his character arc. Happens at times with series - I think of Joe Pantoliano's time with "The Sopranos" as an example.

A lot of people have accused "Lost" of 'jumping the shark' this season. Based on the first few episodes, that case could be made. This was no treading water episode, though, and I look forward to what the producers have planned after the hiatus.