Fenway's Green Monster both helps and hurts batters. Kevin Youkilis provides a great example of why that is. Look at the distribution of Kevin's doubles and homers at home over the previous three seasons:
His line of doubles draws out the position of the wall, with most of his home runs landing in the seats above.
On the road, the doubles to straight away leftfield disappear.
Fenway turns home runs into doubles, but some outs into doubles as well. In this time frame, Youkilis hit 57 doubles and 39 home runs at home, 38 doubles and 36 homers on the road. None of his home runs at Fenway were line drives, however, while six of his road shots went out on a line. He loses a few line drive shots playing in Fenway, but his greater double power more than makes up for it.