I love how even though the movie leans into popular tropes (dad disappointed in song; unpopular guy gets girl; etc.) it subverts the classic happy ending. I like how Hiccup loses his leg, since it shows that winning comes at a cost, and the frame of him walking out with Toothless helping him while his broken tail and Hiccup's lost leg are side-by-side is just very perfect.
This is the first time I've watched this movie since I was 8 or something, so I'm just finding new little things to appreciate that I didn't notice when I was younger.