Lincoln was a capable man and he was in the right place at the right time. The country changed radically during his presidency because of the Civil War. It went from a second-rate power to a world power. This was due to the fact that the North was motivated to pass a lot of reforms while the South was split from the country and the South had to live with all that as an unwritten term of surrender. It also built up the military prowess and ability of the entire country.
To say he freed the slaves is to give him too much credit. The slaves basically freed themselves. The North faced a dilema with the slaves when they conquered Southern territory. If they gave the slaves back, then they were giving back an asset to the South that would help Confederacy with the war effort by alleviating their labors. If they kept them, what should they be used for? Many slaves ran to the North of their own accord and there were close to one million slaves that the North didn't know what to do with (This was the true million man march). The only solution that made sense was to free them. They didn't have the resources during a war to try to place one million displaced slaves and even if they did they had no idea on how to put to use so many displaced slaves, so they let them fend for themselves mostly.
Lincoln's Emancipation Proclamation didn't free anyone. It freed slaves in southern territories where Lincoln had no real power since the Confederacy had control of them. Lincoln saw that the quickest way to defeat the Confederacy was to encourage slaves to stop helping the Confederacy by promising freedom. Eventually in the later period of the war, it wound up being a war against slavery only as a way of defeating the South.
The war wasn't started over slavery. Most white people didn't care since only a small percentage of people could afford slaves. It had little to do with most white people. Ending slavery became incidental and was a war measure to defeat the Confederacy.
Ironically Lincoln was picked to be president because he wasn't too anti-slavery.
The entire reason Lincoln was picked by the new Republican party to be their nominee was that they looked at the 1856 election and figured out what platform their candidate would have to have to win the states they didn't win in 1856 and that they needed in 1860. They needed a candidate that was anti-slavery, but not too anti-slavery. That left out all the prominent candidates (since they were too anti-slavery) and they were left with Lincoln, an unknown who never won a significant public office and just lost for the Illinois Senate two years earlier.
Lincoln won with 38% of the vote and he wasn't even on the ballot in many southern states. Lincoln had a radical approach that he would end slavery slowly by 1900 and put all former slaves somewhere outside of the US. Eventually this was abandoned by Lincoln when he finally realized that the slaves just wanted to be free and would fight against being exiled from what they now considered their home. However Lincoln strongly thought that the integration of slaves into the population wasn't to be desired and it took a lot of time for him to give his plan up.
The Civil War wasn't fought to fight slavery. The South started to succeed as a response to Lincoln becoming winning the presidentacy even though he wasn't even on the ballot in some southern states. The North and South simply saw each other as different with different views, even though they weren't that different. Lincoln's election was just the spark that started the beginning successions.
Lincoln was a very intelligent man who did well with what he had. However he was miserable being president mainly because of the war. "If there is a hell, than I am in it" he was quoted as saying. He physically aged about 20 years in appearance while serving as president.
So he worked extremely hard and he was intelligent and he was in the right place at the right time. He was very capable, but he was also very lucky. At least that's my two cents about it.