My experience being a Program Manager is exciting. I have involved in a number of programs in many countries worldwide. What I like most is being in a position that allows to see the BIG PICTURE how the program is performing in-line to the strategic goals, mission and vision. Like what Budianto mentioned, this is to facilitate top-level decision making, hence the ability to relate well (in my case IT to business)is very important.
A Program Manager has to be good at "seeing the forest" and not the "trees". Only if he/she needs to understand a particular issue well then he/she needs to go down to the "tree" level. The ability to RELATE is very important because things/events don't happened in vacuum. Hence, his/her knowledge has to be beyond the subject matter (in my case I have to go beyond IT). This is where the real fun and challenges is ... the continuous learning.