The easy answer to the question of whether he was a great president is that we'll never know. His presidency was too short to reach any definitive conclusion about its success or failure. If he had survived that day in Dallas, the days, months and years that followed would have determined his ultimate legacy.
Still, those 1,000 days counted for something. Here's my assessment:
1) You can't overlook the fact that the first year of his short presidency was, by his own admission, a disaster. The Bay of Pigs fiasco, along with his anemic performance at his summit with Kruschev in Vienna and inability to launch and substantial legislative initiatives, made him seem weak, uncertain and ineffective. That first year has to weigh significantly on his overall grade.
2) His triumph in the Cuban Missile Crisis, particularly averting the real possibility of nuclear war, easily overshadows the failures that preceded it. The cool restraint he showed during those 13 days, in the face of mounting pressure by the military and Congress to immediately launch a military campaign that many now believe would have led to a nuclear exchange, can't be overstated. He deftly outmaneuvered Khrushchev, along with many within his own government, and found a way to get the missiles removed without going to war. Other presidents in the same situation, namely Eisenhower or Nixon, quite likely would have acted more rashly, with dire results.
Yet, in judging Kennedy's handling of the crisis, one also needs to judge whether other presidents would have found themselves in that situation to begin with. Did Kennedy's hesitance during the Bay of Pigs and in his summit meeting with Khrushchev give the Soviet leader the illusion that he could get the better of his inexperienced foe in a Cuban showdown? Did Kennedy's obsession with toppling Castro and bellicose rhetoric about ramping up the arms race early in his presidency spur Khrushchev to try to take a stand in Cuba to equal the playing field? We'll never know for sure, but it's quite possible that had Kennedy taken a different approach toward Cuba and the Soviets from the get-go, Khrushchev might never have dared put missiles in Cuba and plunged the world to the brink of nuclear war.
3) With Kennedy, you can't overstate the power of words and the man's ability to inspire others to action. With many presidents, it's difficult to identify even one speech that stands the test of time. With Kennedy, there seemed to be too many to even count, and many of his greatest ones came in the last months of his life. Their impact was felt years after his death, and perhaps that is the greatest argument for his greatness.
The American University speech that set forth a path toward detente and peace in the Cold War by casting the Soviets not as heartless enemies, but human beings like us.
June 10, 1963
For in the final analysis, our most basic common link is that we all inhabit this small planet. We all breathe the same air. We all cherish our children's futures. And we are all mortal.
The Civil Rights speech that signaled the death knell of Segregation and called on Congress to, at least, finish the work started by Lincoln 100 years earlier.
June 11, 1963
One hundred years of delay have passed since President Lincoln freed the slaves, yet their heirs, their grandsons, are not fully free. They are not yet freed from the bonds of injustice. They are not yet freed from social and economic oppression. And this Nation, for all its hopes and all its boasts, will not be fully free until all its citizens are free.
The Berlin Wall speech that sewed the seeds for German freedom and ultimate victory over communism
June 26, 1963
Two thousand years ago, the proudest boast was civis romanus sum. Today, in the world of freedom, the proudest boast is "Ich bin ein Berliner!"... All free men, wherever they may live, are citizens of Berlin, and therefore, as a free man, I take pride in the words "Ich bin ein Berliner!"
These words, more so than any of his deeds, shaped the course of national and world history and made John Kennedy, for all his shortcomings, a great president.