It strikes me that Chuck Lorre thinks he's hit upon the next 'Friends'.
TBBT started small and has gotten huge. However, it became quickly apparent to Lorre & Co that in order to manoeveur the show to a more mainstream audience, one must have more girls and thus, make it more like Friends.
The latter half of the last season wasn't as enjoyable. Too much Friya, not enough Sheldon. Too much Amy and Bernadette, not enough of the geeky stuff - which they have notably moved away from. Now, whilst it's an interesting dynamic to have more of the girls, they're not really complementing what made this show great. It's four geeky guys at varying geeky levels. Now we 4 geeky guys, one practically not geeky anymore, dating a hot non-funny, non-complementary smart girl. It worked better with Penny as she was the polar opposite of Friya and Leonard.
Sheldon almost seems like a second thought lately. It's almost like he has to be bolted into the episode to get the laughs. Now, they should have learned long ago - the best episodes involve Sheldon and Penny at the centre.
I may be alone in this, but I was rather hoping to see Penny's former idiot boyfriend (?) become more central. Instead we got Amy and Bernadette. They're cute, but it's diluting this show.