To elaborate on some things that people keep mentioning...
1) Sasuke's change of ideals.
Sasuke has always been a blank page, ready to be coloured in any colour. Obito was succesful in colouring his page into a black, hateful colour. However, this changed when he confronted his Edo Tensei'd brother. Itachi told him the truth, which honestly touched Sasuke to his very core. Sasuke, however, was still is a ruse of his state prior to meeting Itachi, namely a state of hatred, destruction and loneliness.
From this context, he took the words of Itachi and placed it in this scenario. The result: obliterate the Village Hidden in the Leaves.
This ruse slowly dimmed down, however. Itachi said a last goodbye, Orochimaru came and the old team of Sasuke appeared. This took him out of his ruse (started by Itachi's presence and speech) and brought him back to the state of blank page. This time, however, the blank page started from the idea of love, the ideals that his brother left him. He will always be loved by his brother, no matter what.
Starting from this context, the Hokage's showed up. Sasuke got a speech from Hashirama, the strongest person to have ever lived in the shinobi world other than its very founder. Sasuke has always been an "avenger", but this doesn't always mean being bad. As an avenger, being the strongest is what he desires, so that his ideals can become reality. His goal, like he did with Orochimaru, is to learn from the strongest, to become stronger himself.
Now that he met the Hokages, the very leaders of the village his brother loved so much, and got a speech from them, his page was coloured in yet another colour: love.
Now, with the context of the eternal love granted by his brother, Sasuke realised that the village is what matters. The children, the elderly, ... The village is what drived his brother, and the Hokages. It is the very thing that made both his brother AND the Hokages the strongest beings in the Shinobi world.
Now that the revelation had been completed, it could NOT be anything other than accepting and loving that Sasuke feels now.
His change of ideals was NOT a sudden one, it was a very, very long process. One that involved understanding and patience.
2) Orochimaru's change of ideals.
Orochimaru was in Kabuto this whole time. With it, he noticed a lot of different things, different views and different opinions, feelings, emotions, that he would have never experienced if he stayed as Orochimaru. He discovered how damaging his ideals and visions were, through the exact copying of Kabuto, and realised that this was no way to live a worthy life. A shinobi isn't what he used to think. Strength isn't obtained by how he used to think.
Not to mention, Izanagi also had an effect on Orochimaru. He might of have escaped it, but he did understand the basic faults and wrongs he was making, thanks to this technique.
Together with Sasuke's change of ideals, he had the perfect opportunity to change as well. The common feeling of friendship that was shared at that very moment was an extra push for the change for both. Old enemies are new friends, all along.
3) Karin's sudden forgiveness.
She's a girl that is in love. Get over it...