Most of the way through this season I realized I hadn't laughed in several episodes (in the midst of Howard explaining his astronaut training). On the other hand, I was cringing multiple times per episode. I stopped watching. This season makes season 4 look like a masterpiece (it ain't).
There was one particular episode in this season that showcased what was wrong. Leonard and Penny go out together, but they make it clear it's NOT a date: it isn't romantic, they're just "friends." Then they spend the entire episode ripping each other to shreds emotionally. This is simply not Leonard's behaviour: he's a decent guy who goes out of his way to not hurt people. I'm not saying he never does, but when he does it's unintentional. And while Penny occasionally puts people in their place with spectacular zingers, she's never been one for deliberately hurting people either. Their behaviour in this episode would have ensured that they'd never get together romantically again (among other things, Leonard repeatedly makes it clear everything he ever did was just to get sex). The writers have made Leonard and Penny both act out of character in an attempt to create humour, but it was bitter and unfunny and also unlike anything either of them had ever done before so it doesn't fit. And that's how the whole season goes.