Sims can perform a variety of mean interactions with anyone, including their partner. Among these mean interactions are insults, smacking, and fighting.
Your Sims are able to neglect their children (and often will without a player controlling them), resulting in them being taken away by a social worker.
Burning is a death that can occur to your Sims when they fail to cook something in the oven (low cooking skill) or in other potential scenarios. This death can be avoided through the use of sprinklers, fire alarms, calling the fire brigade, among other methods.
When interacting with faulty electronics, a Sim can potentially suffer from a shock, which involves them convulsing in a manner similar to a seizure. These shocks can be fatal, or just singe your sim.
Cheating is an option available in the Sims 2, and there are various storylines with pre-made sims that involve cheating (e.g. Lothario Family, Pleasant Family, etc.). The Romance aspiration encourages the player to make their Sim cheat.
Sims can have mental breakdowns if their aspiration scores get too low and they will collapse on the floor and sit there until a "doctor" comes to "fix" them.
There are issues with some pre-made sims wherein they are written to be related in the story, but the game does not mark them as related, allowing sims to get wants to perform romantic actions with a distant family member. (i.e. Chloe and Lola Singles and the Curious Brothers) Generally, however, the game has a system in place to prevent incest, though I have heard that this system gets buggy in some games if there are too many family generations.
It isn't explicit, but your sims are able to "woohoo" (have sex) in a variety of locations. Visually, there's usually some sort of movement present, usually some smoke and lovehearts. Some noises are also made. There are various "Romance" interactions, and some of them are quite suggestive, like "Goose" (pinching a sim's behind)
Santa is present in the game and follows the established mythos (leaving presents or coal etc). Any attempts to manipulate this NPC outside this purpose (such as moving in with him) will cause problems with the game and possibly corrupt the save.
Not exactly a plane, but when Sims look at clouds or stargaze on the ground, there's a 1% chance they will be struck and killed by a falling satellite. This satellite will crash into the ground.