I already thought the series was getting stale, but killing off Glenn in issue #100 was the last straw for me.
Don't get me wrong; the fact that even the most prominent characters can die is part of what's driven this title to the amount of success it's enjoyed over the years. That being said, at some point, when they've killed off all the characters you like and the same scenario plays out for the fourth or fifth time, it's hard to stay interested in the story.
The group is in big trouble, they find a place that seems livable for a few months, then everything goes to hell during some major catastrophe, main characters die, and the whole thing starts all over.
The camp outside of Atlanta, Hershel's farm, the prison, the suburb in DC; it's played-out, and that's a shame...