I wont necessarily agree on Doctor Who. While I DO agree that the last season were complete trash, I think it could have been saved and refreshed by some better attention to the writing. I think the BBC got bored with it more than anything.
Stargate:SG1 is my prime example series. Once the main goal of the series was complete, the defeat of the G'aold, that is where it should have ended. Essentially starting over with a new enemy and with a story arc that mimicked the first one was a bad choice. I think that rule is a good one to be followed. Start your story, have an ending in mind when you do and build to that ending and then finish it. Better to leave with fans begging for more than to leave with fans thinking your smell like 3 day old fish.