The reason Big bang isn't interesting anymore is because none of the writers want the characters to grow. Sheldon who once used to come in the ranks of House, Sherlock Holmes and Tony Stark is now lowered down to a blabbering buffoon. In fact every character had become a stereotype of themselves. Howard and Bernadette are dealing with parenting and most of the so called "jokes" come from her bossing him. Raj spends half the season failing in his attempts to get a girl and the rest is about his bromance with Howard. God knows why Amy is still with Sheldon when she spends 90% of the time bickering and complaining about him. And 90% of the time the episode ends with him saying something sweet to her and she forgives him. (If that is not calling a character shallow then what is?). Penny and Leonard's stories have exhausted. Penny is only left with jokes which include her making fun of geeky terms.

The characters descriptions sound so single-dimensional because they are single-dimensional. No idea what part do people find funny. Clearly they are watching a different show than me.