The Big Bang Theory and how it has changed over the 4 series( yes I am talking about the TV show)

Even though I live in Britain I don't find any of the British sitcoms appealing to well as mostly they are  aimed at the working class which needs basic drama and horrendous jokes to be entertained, apparently. So when I watch any TV show it mostly originates from across the pond. I find the American show sometimes more professional than the likes of EastEnders or whatever backward little show the BBC has produced. This leads me to discuss my beloved show called The Big Bang Theory.

You see what most captured my interest in BBT is that it had a range of themes in the show and it wasn't all about sex and how to find the 'one'. At the beginning it had a big input of scientific merit in it which may be why it appealed to me. However it also had the 'dating' input which I also enjoyed because it was played simultaneously with the science and it just mixed and that made it so different and so better than other mainstream sitcoms. However in the last series the writers decided to include multiple serious emotional themes and yeah it was still funny but it was just different. And slowly it's losing its appeal for me and no great acting from Jim Parrsons is going to change it.

Do any of you know any other shows that have slowly changed and lost it's appeal to some of you?
Let me know.