Some characters are seen kissing on several occasions, some of them gets interrupted though.

In several episodes, pants are lit on fire, usually on the butt, but underwear is never shown.

Some mild innuendo. For example, a character "flirts" with his biceps and calls himself "daddy" in relation to the biceps flirting.

References to bowel movements and genitals are often said, but usually interrupted before it could be taken too far.

A female dragon is seen exhausted after she lays an egg. The actual birth is not seen.

Tuffnut mentions that he bathes in open water, everyone looks at him in disgust.

Hiccup and Astrid are seen laying on top of each other in a couple of episodes, usually either by accident or while training.

There is quite a bit of implied sexual tension between Hiccup and Astrid, especially during the final episodes of season four and in the following seasons.

In at least one scene a character is implied to have been going to the bathroom, though nothing is shown.

Different scenes of main characters fighting and beating each other up. Violence is not meant to be implicated in these types of scenes

A villain orders another villain to be executed motions this by dragging his thumb across his throat.

After falling into a volcano, a man gets his face burned by a sudden burst of lava. His scar can be seen as graphic.

This show is definitely darker than its predecessor. Characters die both on and off scene in violent manners (one is frozen alive and another drowns), dragons are imprisoned and often killed (although the latter is never seen), and there are many action sequences that can be graphic to some.

Human bones are seen lying on the ground in one episode, including a couple of skulls. Heavily implying they might have been eaten by a large dragon and its pack.

A captured enemy runs into some bushes and is later heard screaming afterward. The screaming stops a couple of moments later and dragons appear, implying they killed and/or ate the man.

One of the series' primary villains has a visible scar on the side of his neck that appeared to be caused by a dragon.

A villain has a large burn scar that covers nearly half of his face, he also has a blind eye.

In a few episodes, dragons are heavily scarred and a couple of them are close to death, but they all survive. Dried blood and open wounds are usually seen on them, one dragon suffers from a severe burn on its back.

In the episode "Crushing It", Tuffnut greets Stoick by saying "Welcome to Rumblehorn hell!"

Mild profanities are uttered extremely rarely. The harshest profanity ever said is the word "crappy" in the episode "Snotlout's Angels".

The show features "Viking" tailored "expletives" such as "Oh Thor" in place of actual swearing. Some references to bodily functions or implied loss of bowel control.

None seen or mentioned, although people can be seen drinking out of pints that could be filled with alcohol, but there is the possibility that they're not.

The episode "Triple Cross" is by far the darkest episode of the series. Viggo's death is very upsetting, especially considering he sacrificed himself to save Hiccup's life.

Heather threatens to and nearly succeeds in killing Dagur several times, even after he's reformed and revealed to be her brother.

A funeral pyre is given to diseased sailors at the end of an episode. Astrid's tone during a speech just before it happens is very depressing.

A villain falls into a volcano, this was an intense and unexpected scene. He's revealed to be alive some time later with a large scar on one side of his face.

One of the main villains holds Astrid hostage and threatens to kill her in front of Hiccup.