The play itself doesn't translate well into the modern age. It trades on the satirical views of Americans as simple-minded and vulgar, but also honest and blunt about the hypocrisies of European manners. The selling points were the various ad-libs allowed to the minor characters such as Lord Dundreary, whose mixed-up bad homilies became known as "dundrearies"
But it was popular for the day, and akin to the modern audience's willingness to watch a half-decent Adam Sandler repeat on the Comedy Channel, Lincoln and others would re-watch repeat performances.
Lincoln was in a good mood anyway. The war effort was winding down as Lee's surrender was the death-knell of the Confederacy's fighting spirit. While the ongoing efforts to plan a reconstruction to bring the rebel states back into the Union were looking messy, there was a lot to look forward to. He had plans to travel the length of the nation, to be the first President to view the Pacific coast from California, to be witness to the efforts of rebuilding the United States through Homestead-granted lands and pioneering citizens (in more fanciful what-ifs, Lincoln could well have visited San Francisco and met the Emperor Norton: there's evidence the two had exchanged letters...).
Unfortunately, the word that Lincoln would attend that evening's show got to an actor (I refuse to name him, although it's already too well-known) familiar to that theater (he was not a member of the cast) who also happened to be a Confederate sympathizer with a local circle of allies. Seeing an opportunity to avenge a defeated Confederacy, he schemed a quick plan to act out his fantasies of being a national hero at the deadly expense of the nation's leader.
Which becomes one of the most macabre jokes in American history: Other than that, Mrs. Lincoln, how was the play?
The loss of Lincoln at the key moment of our history becomes one of the disasters that haunt us to this day. Instead of a Republican with pragmatic and long-range visions of the national dilemmas over race and poverty leading our nation, we ended up with a Democrat in Andrew Johnson whose political ambitions drove him to institutionalize racism and allow the Southern political powers to retain control.
As much as we need to remember Lee's surrender at Appomattox, we need to remember how the loss of Lincoln tainted our nation's chances to build a stronger Republican that lacked the racism and hatred we cope with to this very day.