Aside from that one quibble, I generally agree with the grades Bill Easterly and Chris Blattman accorded to Obama's speech in Accra. I'd probably only give Obama an A-/B+ myself. 

The reason is less substance than the optics of it all.    

To be sure, a speech before Ghana's political elite is a smart choice for a number of reasons.  It is a fine reward for the political maturity Ghana's elite exhibited in the wake of a tightly contested election that was decided by less than 1 % of the vote.  The ruling party lost, but rather than rail against election irregularities, it gave up power.  The peaceful transition of power from one party to another is all too rare on the continent and Ghana's political class deserves praise. However, I get a sense that in service of rewarding the Ghanaian political elite, Obama missed an opportunity to speak directly to the people. 

I happened to be in Addis a couple of weeks after the elections.  The excitement over Obama's victory was evident nearly everywhere you looked.  A teenage kid hanging outside the main UN headquarters was even hawking bootleg DVDs of Obama's Democratic National Convention acceptance speech. Apparently, they were selling.  I bought myself a copy of Dreams from My Father--in amharic--from a street vendor nearby. The title's translation, an amharic speaker told me, reads "Secrets to Greateness and Change."  

This anecdote and others I have heard strongly suggest to me that the President of the United States may be the most popular political leader in Africa.  To that end, I think the speech would have been more effective had it 1) occured in a public setting, like a public square or stadium and 2) drawn more from Obama's signature, direct-to-the-people inspirational oratory.     That's the reasoning behind my A-/B+.