It's interesting though that most votes (even mine for "Richard Mylan for Richard Coyle (Coupling)") are based on the genius of the previous actor, not necessarily the lameness of the replacement. Take "Ben Browder for Richard Dean Anderson" - Ben Browder is a fantastic actor, was brilliant in Farscape and I thought he was really good in SG-1 (especially when Claudia Black guest starred and we had the same great on-screen chemistry), but there's just no way he could reach RDA level of awesomeness. RDA was the heart and soul of the show and made it worth watching. Ben Browder just couldn't do that. Just like nobody could rival the brilliance of Richard Coyle's Jeff. As much as I enjoyed Oliver Morris, he just wasn't a Jeff.
Others are more interesting. For example Spin City: I love that show and I think Charlie Sheen did a marvelous job, but right around the time he replaced Michael J. Fox the show kind-of jumped the shark and together with him changing the way the office worked and felt it was just too much. Duchovny and X-Files is a very similar story, only it combines jumping the shark some seasons prior with him being the heart of the show.
"We're on the side of the demons, Chief. We're evil men in the gardens of paradise, sent by the forces of death to spread devastation and destruction wherever we go."
-- Colonel Tigh
