Sinon was definitely a good addition. Kirito needed a friend who wasn't another SAO survivor and she was strong (in the game anyway) and fiery and it fit her role perfectly. I loved the action and I enjoyed seeing Kirito adapt his in game sword skills in order to use them in a gun game and succeed. One of my favorite scenes is when he cut Sinon's hecatete bullet in half. They also made a good and took care of Death Gun. Kirito also helped bring Sinon out of her shell in the real world.
The Calibur arc was short, but enjoyable. It was nice to see a story arc based around Excalibur after it was previously introduced and I really enjoyed the boss fights in the arc. The Mother's Rosario arc was great too. Asuna is my favorite female in the show so I like how they showed more development for her. The addition of Yuki and the sleeping knights just added some story drive to the the arc and the guild had a huge impact on Asuna. Seeing the way her mother acted at first helped explain why she had an aggressive attitude sometimes. For how much I love the Aincrad arc, I think season two was better in the fact that every arc served a purpose; the Fairy dance arc did, but some of it was unnecessary. Due to how great the Aincrad arc was though, I'd say both seasons are equally amazing.
The music is always fantastic along with the art as well.
