I know this thread is a couple months old but I just finished season 3. I was NOT impressed with Matthew dying, but should have seen it coming with how cheesy he got in the last episode, heck the whole season. I didn't like him much in season 3.
I think Obrian should leave. She's done so much and has been involved in so many plots on the show that I don't what else they could do with the character.
I love how honest Mary is about her being bitchy. Hehe.