If you have been playing Sims 3 for very long, you have probably noticed that there are not as many ways to kill off your Sims as there were in the previous games!  This is partly true because each expansion pack introduced new ways to die, so that collectively if you owned every expansion pack for Sims 2, there were at least a dozen possible deaths.

The Pool (Drowning)

You may also have tried to drown your Sims, only to learn that they can now get out of the pool even if there isn't a ladder!  They just climb out the edge!  I literally laughed out loud the first time I saw this happen.  Methinks the EA developers are having a bit of fun at our expense.  I wonder how many Sims drowned in pools over the years?  It certainly must have been one of the top causes of death.

Your Sims actually can still drown in the pool, but it takes a bit of intervention on your part.  As soon as they get into the pool, switch to Build mode.  In addition to deleting the ladder, build a wall right around the perimeter of the pool.  Thus unable to escape, your Sims will drown.

The Very Small Room (Starvation)

It's a lot harder to starve your Sims to death, too!  On the up side, Sims are a lot less likely to let themselves starve to death.  (I can't count the number of college roommates I lost in Sims 2 University because they kept complaining they were hungry, even though there was a fridge full of food right there.)

If you simply delete the fridge, your Sims will not voluntarily leave their home lot without your direction.  It also seems that they will not spontaneously order pizza using their cell phone.  However, it takes them a LOT longer to die of starvation than in previous games.

I'll be honest, I only had the guts to starve a Sim to death once.  It was a pretty horrible experience all around.  I don't think I could do it again.  

Electrocution

Sims with low handiness scores who are instructed to repair a broken object have a very small chance of dying by electrocution.  (This probably wouldn't happen as much if they didn't try to fix it by sticking a screwdriver in there.)  I've tried this repeatedly, but all that happens is my Sim gains handiness skill, eventually fixes the object, and might occasionally get shocked/singed.

Fire

This one is obvious!  Stand your Sim near something that's caught on fire.  You can also start a fire by having them cook something in the oven, and canceling the action before they take it out.

Sims can also catch on fire and die when they blow out the Birthday Cake.  It's rare, but the name "Birthday Inferno" is kind of a tip-off, no?

World Adventures: Mummy's Curse

Visit Al Simhara, awaken a mummy, then lose a fight!  Your Sim will be cursed as punishment for losing, and will die within two weeks.  In the last few days of their life, the screen will turn dark at the edges when that Sim is selected, as if their vision is slowly clouding over.  (Dramatic!)