The US relationship with Cuba is one of both countries' longest standing antagonisms. Cuba has been one of the epicenters of concerns about American security in the western hemisphere since the failed 1961 Bay of Pigs invasion. The once-powerful Cuban military has been on the other side of quiet conflicts from American "advisors" and allies from Nicaragua to Angola. Most famously of course, the Soviet Union's attempt to install nuclear missiles in Cuba was perhaps the closest the US and USSR ever came to World War 3. Now, though, Cuba is a political and economic backwater. While a few Americans and many Cubans stand to become much more wealthy through renewed relations, the main benefit of normalized relations is likely that American politics will be able to forget Cuba. Sure, Havana will blossom back into a go-to destination for tan lines and daiquiris safe from the cartels, future baseball stars will be able to sail to Florida much more comfortably, and bartenders across the US will likely be frustrated by a newfound appreciation of Havana Club and properly made Mojitos...but never again will Cubans and Cuban issues dominate sectors of American politics the way they have for 50 years.
This renewal of relations could be a sign of things to come in negotiations with Iran. If President Obama and Secretary Kerry were able to negotiate a renewal of relations with Cuba, they may be able to get a meaningful deal with Iran. They'd piss some Republicans off in the process, of course, but the Obama administration is a lame duck and may not care. If the message is spun right, being able to conclude a deal with Cuba AND Iran could be public relations gold come 2016 and Republican intransigence on the issue would be a strong call to Democratic voters.
Unlike Cuba, however, a deal with Iran wouldn't just be an exercise in ditching the stale remnants of a Cold War policy that should never have been continued past 1992 except that Cubans are the swing vote in a critical swing state. Iran is now what Cuba was 40 years ago: an antagonist on the world stage, a potential nuclear threat, and the shadowy funding and advisors behind some of the non-state actors the US is engaged in quiet wars against. Iran is also a much larger country, with the potential to be a major economic force in the Middle East, which despite various historical antagonisms shares a number of natural foreign policy priorities with America.
Iran has the potential to be a critical US partner in a number of its foreign policy priorities for the Middle East and even Europe, especially if their rivalry with Saudi Arabia can be quietly managed. They could supply gas and oil to Europe through a pipeline across Turkey (the cancellation of South Stream reopens the Nabucco question), they could provide a counterweight to Islamic extremists across the region, and put pressure on Hezbollah and the Assad government to tone down human rights abuses. Considering the compounding economic hardships that Iran is facing, and the now-proven boldness of the Obama administration thawing frozen relationships, concluding a meaningful deal with Iran is as likely now as it ever has been (well, ever since things went bad anyway). This would be a major advance for US interests in the region and should be considered a top tier priority.