Not as Good as the First One
I honestly thought this movie was average at best.
Drago Was a Lousy Villain: He was nothing more than a generic doomsday villain. He's also a total dumbass. So, he has all of the dragons and the heroes are mourning Stoick's death. What does he do? He tells them to meet him at Burke. "I could just blow you all up with my dragon army, but I won't, because I'm an idiot. Bye."
The Plot Hinged on Deus ex Machina: Drago can mind control dragons... just because! Toothless can break the alpha dragon's control just by Hiccup's words and suddenly has a super mode... just because!
Hiccup's Mom: The previous movie implied that she was a warrior like Stoick, and yet this movie reveals that she's a pacifist. Why didn't she return home with her army of trained dragons? The reason why Stoick didn't believe Hiccup about Toothless in the first movie was because he attacked the other Vikings to defend him and he was too reluctant to show him. Valka could have easily showed the other Vikings what she could do.
The Love Triangle: The whole dick-measuring contest between Snotlout and Fishlegs to get in Ruffnut's codpiece added nothing to the plot and got resolved in an incredibly rushed manner.
Trailers Always Spoil: The previews basically told you the whole movie, particularly Hiccup's mother being alive and Drago controlling the dragons.
The Death of Stoick: The movie made it so obvious he's going to die. The second they talked about Hiccup becoming the chief, I immediately thought "Stoick is going down."