Maturity in the characters destroyed all the humor
The first three seasons of this show are the worth to watch. In fact, this show ratings and success is based upon these three seasons. These seasons are mainly about comedy with little to none emotions. But as the show progresses and Leonard, Sheldon and Howard finds their life partners, whole tune of this show changes. You'll find romance, love and emotions with some dose of comedy after that. I think at this point, this show loses its main theme. I am rating it 5 only because of the first three seasons. The quality of show will start to decline gradually after season 3. But i think it shows good depiction of scientists. I also have some scientist friends who have many similarities with the characters of this show.