'Think of' suggests a brief overview of the topic, where 'think about' invites more considered detail:
'What do you think of Michael Jackson?' 'I think he's pretty strange.'
'What do you think about Michael Jackson' 'Well, I used to really like him when he was part of the Jackson Five, but I think he's gone through a lot of wierd changes over the years. His recent run-ins with the Morals Squad make me think that he can't be entirely innocent of some of the charges laid on him...'