How so? Moving the highest marginal tax rate down from 90%? I don't think that there's anyone (outside of people like opebo) who would support return to Eisenhower-era tax rates.
You'd be shocked. Most liberals think you can tax as much as you want without harming the economy. Witness the Krugman nostalgia for 1970s tax rates.
His actions during the Cuban Missile Crisis? Hardly controversial by any standards.
The Castro-apologists would whine that he opposed a heroic, nationalist leader of the masses who was only attempting to liberate his people from U.S. imperialism.
Championing (though he died before it could pass) Civil Rights reform? Excellent measure, and hardly controversial on the left.
Liberals on this board are already complaining that Obama won't support gay rights enough. Could you imagine them watching Kennedy basically urging reformers to slow down on desegregation?
Sending "advisors" to Vietnam/assassinating Diem? Considering how few people knew about either (the war there didn't gain a significant American presence until '65) I can't see how it would "shock" many people. Besides, initially, there wasn't anything wrong with holding to the Kennan Doctrine and supporting the South Vietnamese regime. It's true that Kennedy's final actions there, signing off on Diem's murder to put a more America-friendly puppet, was reprehensible, but Diem was a real piece of work, anyways, and the images of self-immolating Buddhist monks were seriously detrimental to American commitment in Vietnam.
Liberals pretty much oppose anything associated with the Vietnam war. Besides, didn't Kennedy say that we would "pay any price, support any friend, oppose any foe in order to ensure the survival and success in liberty"? Could you imagine any Liberal accepting that "neocon" position today?