Not that the main plot on a TV show or movie was bad, but times when you thought the background had more of your interest. Use the spoiler button if you want to get more specific.

the brady buch had a number of eps where the subplot was better than the main story

Here comes an example from a drama series: "Dr Quinn, Medicine Woman". The major plot for two episodes is that Michaela, Dorothy, Grace and Myra are on a hiking trip towards a mountain peak. Yawn... I so prefer the sub-plots about the men planning to build a hotel (Sully and Robert E are the only ones, who wants to stop it) and Brian and Colleen taking care of baby Samantha.