I know it's about quality, not quantity, but when you see a 4 for 1 trade, I don't think many people would expect the 1 to be Hunter Pence.
I am only vaguely familiar with the centerpieces that the Phils sent: Singleton and Cosart. I think at one time Singleton was said to be untouchable. Oh well. Some folks say they overpaid, some say they stole. I'm not a prospector, but my uneducated eye says they overpaid. The Phils are already on pace for 100+ wins. Pence is not adding anything to that, and he won't match Jayson Werth's production. This was a move for '12 and '13.
So Domonic Brown remains untouchable. At the beginning of the season, Ruben Amaro said 2011 was not the right time for Brown to be playing everyday in the Majors. He was right, but they had nothing at all going on in RF. Now, Dom will need to be sent back to AAA so he can get everyday ABs. Then, next season, he'll presumably be the everyday left fielder.
As Mariotti points out, he comes to us well versed in being a Cub, what with coming off a season shortening injury, surgery, and oh-by-the-way, his team won the championship while he was out with an injury.