The prophecy of the "Warrior in Black and White". While Po is certainly the most obvious candidate who fits the mold perfectly, there is another great warrior of black and white who could qualify just as well: Shen himself. After all, it's only after his attempt at genocide that he was exiled from his home. He also died at his own hands, intentionally or not. And while Po did effectively destroy Shen's army, he sure had a lot of help from all those cannon balls that Shen kept sending his way, even after witnessing then and there how Po was directing them back at him.
What finally stuns Lord Shen at the end is notPo countering cannonfire with kung fu to singlehandedly destroy his fleet, but the fact that Po is neither permanently mired in a Heroic BSoD believing the lie that his parents abandoned him because they hated him, nor charging in a Roaring Rampage of Revenge for the extermination of his people.
When Po discovers what Lord Shen did to his parents, and for that matter species, his response is to... let go of his pain and find inner peace. Shen, who has never got over his own deep-rooted issues, is utterly shocked.
Played with when the Soothsayer and Shen first meet Po. They know that Shen attempted genocide against the Pandas, and assume that the reason that the Panda is here is to a) avenge his family/species, and b) destroy Shen once and for all. Both are shocked when they learn that not only had Po never heard of Lord Shen, but had no idea why he should have. The look on Shen's face when Po just brushes him off, totally ignoring the well-prepared speech upon their first meeting, is priceless.
Big-Lipped Alligator Moment: While trying to convince Masters Ox and Croc to escape the prison, Po delivers a Rousing Speech about how they will defeat Shen and destroy his weapon. A random prisoner then enthusiastically agrees, but he's never seen again. Oddly, he was a boar and had a completely unique character model. It turns out that this character was going to be in the movie as a fourth master (alongside Ox, Croc, and Rhino), but was cut for one reason or another and the character model was used in the place of a random criminal.
Contested Sequel: Opinion is pretty strongly divided - the first movie is generally regarded as having a stronger structure and a classic theme, while supporters of the second point to superior characterization and animation. That said, it seems more downplayed than most cases, since fans generally like both films.
Lord Shen. Any surprise? Driving the point home further, he tends to get the Bishōnen anthropomorphic treatment in fan art.
The Wolf Boss has also been subject to this. Despite his prominent role in the panda genocide, plundering of innocent villages, and direction of the goon squad that terrorizes Gongmen City, some people portray him as a cuddly puppy. He's a goofier character than Shen, but he's far from an Anti-Villain. Even him turning on Shen was a case of Even Evil Has Standards, not any remorse for what he's done under his command.
Ensemble Dark Horse: The Soothsayer. Much like Master Oogway in the first film, she's utterly beloved for being an incredibly wise, funny, and yet deeply human mentor figure, and her one-on-one scenes with Shen and Po are some of the best in the entire movie.
Evil Is Cool: Unsurprisingly, Lord Shen's many fans definitely think so.
Po's Oblivious Adoption, joked about in the first film takes on a darker note once we learn he's a genocide survivor.
The end of the movie confirms that Po's father Li Shan and a handful of pandas survived the onslaught. It provided hope that his mother did as well. Sadly, Li Shan confirms that she was killed in the attack.
Heartwarming in Hindsight: Li Shan fighting off the Wolf Boss to protect his son and wife is even more heroically badass when the third movie reveals that he has no knowledge of kung fu whatsoever... and went up against Shen's forces armed with nothing more than a father's love.
They Wasted a Perfectly Good Character: Addressed by the writers themselves with the Soothsayer being voiced by Michelle Yeoh, a skilled martial artist, while her character is one of the very few never performing any martial arts in the film. They considered it justified seeing as the Soothsayer is an old lady, but still found it an ironic casting.