I've technically only been watching "The Big Bang Theory" regularly since the start of this season, but I've seen every episode, and while I'm not ready to call Thursday night's episode, "The Toast Derivation", the worst episode ever, it was highly indicative of the overall decline in quality in the show this season.
The introduction of the new female characters has clearly affected the show's overall dynamic, with almost all of the main characters acting significantly differently than how we first met them. It's most noticeable with Sheldon, and this episode emphasized those differences. The episode centered around the shifting social dynamic in the group, with everyone hanging out at Raj's place (so Leonard could spend more time with Raj's sister, one of the many, many women who have been inexplicably irresistibly attracted to him this season). The Sheldon we were introduced to in the early seasons would have been fine with this, even as a variation in his routine, because that Sheldon saw social interaction as something forced on him by society, and a distraction from his work/overall genius.
The scene at Raj's with everyone talking about Sheldon almost felt like a wake for the character, like they were all reminiscing about someone who was no longer part of their lives, and in a way, that's true, because Sheldon isn't really "Sheldon" anymore, and the character in his place isn't nearly as funny as the one that used to be there. On top of that, the show is now overstuffed with characters, resulting in some painfully unfunny scenes of Penny, Amy and Bernadette having a "girls' night" and jokes about electric toothbrushes. Considering this show used to get mainstream laughs out of physics jokes, this seems like a huge step back.
Now, as I mentioned, I technically haven't been a long-time viewer of the show, so I won't feel as bad if the decline continues and I have to drop the show from my schedule, but if we get more episodes like this in Season 4, I may not be back for Season 5.
For more coverage of the 2010-11 television season, visit the AdamReisinger.com TV Reviews home page.