The normal genre elements
Do not get me wrong. I love me some Godzilla. But I am pretty sure a giant Tokyo monster stomping down the hallowed halls of Silverton is pure Ether Saga Online Gold genre. I am not a member of the pure-sword-and-magic camp, but I am still positive having the named actual Godzilla is outside the norm. Crossing genres is a pretty big sign that your campaign is jumping the shark. It happens because the creative personalities are struggling to find something new and different. All the stories are getting repetitive; all the normal genre elements have been used up. You go outside the genre to find something a little fresher.