You can read the announcement, which focuses on Silverman's successor, here. While the network has had some shows I like very much, including "The Office" and "30 Rock," overall it has been disastrously unsuccessful in recent years. I mean, "Kath & Kim"? Revamps of "Knight Rider" and "Bionic Woman"? And this Jay Leno prime-time move in the fall is a nod to penny-pinching and an exhausted creative process. See stories here and here. The second link has a great graphic:
It also has a list of the shows NBC aired on Silverman's watch, which is awe-inspiring in all sorts of bad ways.
(Hat tip to TV Tattle)