I disagree with the accepted answer. You can "think of" something that is important and detailed.
Newton thought of a way to explain the workings of the solar system.
That is no "cursory glance" at the topic, but the result of a sustained and reasoned effort to create a theory.
Similarly, to "think about" something may not mean to think in depth.
Drunk and giddy with pleasure, we thought about our days together as children.
This may mean a series of random associations, not a substantial act of mental labor.
To my way of thinking, there are so many variations on the meanings of "to think of" and "to think about" that it is impossible to draw distinct lines between the two terms. Sometimes they are synonyms, sometimes they are something else. Take this example from Wallace Stevens' poem, "The Sense of the Sleight of Hand Man":
To think of a dove with an eye of grenadine
And pines that are cornets, so it occurs,
This is both a trivial application of "think of" and a profound one. It is a fleeting passage of imagination but also a deep epiphany. It is much more than the same construction used in a sentence like "I'm thinking of a number between one and ten."
You can "think about" something deeply or shallowly. Usually it means to consider something, but occasionally it means something different. In the following sentence, it means cursory attention was given to a notion:
I thought about saving some of the ice cream for my sister, but decided to be greedy and eat the rest instead.
Yet "think about" can be profound as well. In the screenplay to the film Mystery, Alaska, John (played by Russell Crowe) confronts Charles (Hank Azaria), the promoter of the big hockey game:
You brought back the one thing that could tear the heart out of this place. Now, either you thought about that or you didn't.
This is a stunning, multi-layered accusation: If Charles had actually thought about the risk and proceeded in spite of it, he was someone who put personal success above the good of his hometown. But if he hadn't given it enough thought, he was either stupid and insensitive or the welfare of his hometown didn't matter much at all.
In summary, both expressions are capable of nuance and affected by context. There is no clear, cut-and-dried distinction to draw between them, no simple differentiator that will work in all cases.