Sorkin, unfortunately, made the same mistake here as he did with Sports Night, only faster this time. He turned away from a fun, smart buddy show and instead, focused on dysfunctional, hurtful relationships. I loved every episode until the Matt and Harriet relationship starting dominating. Then the show lost it's momentum. The same with the Casey and Dana relationship in Sports Night.
In both shows, you have a cast of characters with great chemistry. They lead exciting lives and are exceptionally good at their jobs. It doesn't take long for us to like and admire these characters. With that, you can keep an audience. That is, unless you leave the audience feeling bad for their favorite characters at the end of episode after episode. I don't tune into shows to feel worse when its over.
The last episode of '60' went back to the starting formula. It showed they knew how to write it, but chose not to.
It was too good a show to let die so soon.