I thought Shiro's disappearance last season, injected a little bit of life into the show, forcing them to change the dynamics, adding more development to Keith, Allura and Lance (which is good, since Lance is my favorite character) and Shiro's return added a quite welcomed can of worms since we don't really know what happened when he was recaptured by the Galran empire. I also liked the added backstory and humanizing of Zarkon and Witch Haggar since I thought one of the main weaknesses of the first two seasons were that the Galran Empire wasn't all that interesting.
However, the best part of the season was by far the addition of Prince Lotor and her quirky mini-boss squad. They're very competent, more interesting and seem a little bit more of a threat to Team Voltron than his father. Their dynamic and role reminds me a bit of Azula, Man and Ty Lee on A:TLA, even if they're not as great as characters as the Fire Nation trio (at least not yet).