Raj is a creepy idiot, Amy's social anxieties and apparently hilarious, plainness of appearance is played for laughs, Howard is the most archetypal nebbish Jewish guy imaginable (even with his sweet 60s Merseybeat bass player hairdo), Bernadette is a dizzyingly unoriginal turn around the sweet-but-actually-crazy-woman trope, Sheldon has so many tics and 'quirks' that there's no room for anything else, and Leonard is so boring I was actually sat here trying to remember the bloke's name for a good three or four minutes.
They're all grotesques except Penny, and everyone mocks the fact she's slept with more than one guy ever and the fact she doesn't have a PhD. While sitcom as a genre is at least partially powered, comedically speaking, by the situation in which the characters find themselves in, what makes a sitcom something you return to again and again is some sense of connection with the characters.
On good sitcoms, that sense of connection comes from a shared way of seeing the world (like any and all of the main Friends cast), or a particular life trajectory which resonates with your own (like Jess from New Girl, for instance), or an intriguing contradiction at the heart of their character which gives them greater depth, believability and longevity (like Eddie the dog from Frasier).
The Big Bang Theory lot are much more a collection of rapidly-thinning character traits with arms and legs than anyone you'd take any pleasure in spending half an hour of your life with.