First of all I'm going to talk about the actual bluray transfer of the show, the first 1 or 2 seasons weren't brilliant transfers and it wasn't hard to notice them getting better as the series progressed and technology advanced, but this season is leaps and bounds ahead of the last five in terms of picture and sound quality. I usually use the eyes (like camera operators in the film and television industry) to see how clear the picture quality is on a bluray and usually you get a little bit of grain around the eyes and in the background, but the transfer is top of the range, there is literally not a grain in sight (watched on a 52" flat-screen).
Now I've seen people complain about the quality of the series and a lot of people saying the genius of the show is in the writing and Jim Parsons of course, while that still holds true to the day, characters like Rajesh and Bernadette really get fleshed out and improve dramatically this season, Amy Farrah Fowler is a gem of a character who is literally made by the brilliant Mayim Bialik, if Mayim wasn't portraying that character I have no earthly idea how she could succeed.
The bottom line is this is one of the hottest shows on television right now (up there with The Walking Dead, Game of Thrones and Breaking Bad) and this season shows us exactly why, episodes like "The Spoiler Alert Segmentation", "The 43 Peculiarity" and "The Parking Spot Escalation" are true examples of what makes a sitcom great. This is definitely the finest sitcom to come around since the greats of Friends and Seinfeld.