I guess what can be said about The Big Bang Theory is that it has gone steadily downhill for the last season and a half. The third season was a big (and a rather unexpected) disappointment. The fourth season, unfortunately, has been even worse.
There are of course numerous reasons for the decline in quality, but I'll concentrate on the five big ones. These are:
1) Amy Farrah Fowler's character.
2) Sheldon degenerates even more.
3) The characters aren't underdogs anymore.
4) Lack of antagonists.
5) Writers don't know how to write Penny and female characters.
1) Amy Farrah Fowler's character: I don't know what the writers were thinking when they decided to create another Sheldon in Amy Farrah Fowler's character. She doesn't work at all.
I have nothing against Mayim Bialik as an actress. I liked her in Macgyver and as Blossom when I was a kid. It's just that on TBBT her character creates a black hole that sucks everyone else inside with her.
But why is AFF's character is so bad? Well, The Big Bang Theory is (or was) supposed to be about men vs women and geeks vs normal people. Every character should be an outcome of these four character attributes.
Mayim Bialik's character unfortunately doesn't follow these guidelines. In fact, her character isn't defined at all. We don't know if she's a geek or a normal person. And to be honest, do we even know if she's a man or a woman? She's just sitting there doing absolutely nothing.
2) Sheldon degenerates even more: I wouldn't have believed after the third season that Sheldon's character could get any worse. But that's what happened. Sheldon doesn't have any personality anymore. He only shouts out caricaturish lines that don't have anything to do with his twisted idealism.
If the writers did happen to get an idea where Sheldon has idealistic thoughts (like becoming a robot) the whole thing turns into a very ugly and lazy slapstick. Sheldon is supposed to become more human as people with asperger's tend to do. He's not supposed to regress.
3) The characters aren't underdogs anymore: Everyone roots for the underdogs so there's no good reason not to have them on the show. However, the writers went to silly lengths to make sure that none of the characters were in a 'down' situation.
Howard having a girlfriend is ok, since Bernadette is a funny character. But Leonard's conquests this season have been awful and have come out of nowhere. The most unfortunate instance of 'no underdogs'-theme has been Raj having a deaf girlfriend - off-screen. That's simply wrong.
4) Lack of antagonists: let's count the best antagonists so far on the show: Leslie Winkle, Dr. Gablehouser, Stuart, Wil Wheaton, and Leonard's mom. How many are left? Well, none basically. Wil Wheaton had a quick but unnecessary cameo in the Indy episode. Others have been missing in action.
When you take strong antagonists out of the equation, you're not left with much. It shouldn't be that hard to come up with story ideas where someone stands up to the guys. I myself managed to write a spec where Dr. Gablehouser shows Sheldon who's the boss, so it obviously can be done.
5) Finally: Writers don't know how to write female characters: This wasn't really a problem during the first two seasons. The characters were new so in certain ways Penny's character wrote itself. Now that we know the characters, we should get more insight into her.
Too bad that this doesn't happen. Instead she gets dumber and meaner as the seasons go further. She doesn't seem to have any ambitions whatsoever. Someone pointed out it might have something to do with those pesky middle aged divorced men on the staff...
Speaking of women. Nicole Lorre (presumably Chuck's daughter) has come up with the funniest storylines up to date: Barbarian Sublimation and Vegas Renormalization. I wonder what's she's doing on the show nowadays.
In conclusion: 11 episodes behind: 1 that was funny. No wonder I have become more and more afraid of watching the next episode. It might be funny, however the chances unfortunately are that it isn't.