There are fewer ways to die (so far) in The Sims 3 compared to Sims 2.  For one thing, the designers have introduced the world-shaking ability for Sims to get out of the pool without using a ladder!  They just hop out the side!  UNFAIR.  (But it's okay - if you want to drown a Sim, just build a wall around the pool so that they can't climb out.)

In fact, there are only five ways to die in Sims 3: drowning, electrocution, starvation, fire, and old age. By comparison, there were almost two dozen forms of death in Sims 2, ten of which shipped with the base game.  There was also a lot more variety in post-death drama. In Sims 2, if you died of old age and your lifetime happiness was high, you would be met by Hula Death.  The Grim Reaper wore a lei, and was accompanied by two ghostly Hula dancers.  As far as I can tell, in Sims 3 the only difference between deaths is whether the dead Sim begs for their life or not.  

Speaking of which, there also does not appear to be an option to plead with Death to let the Sim live, as there was in Sims 2.  This got me out of a real pickle once, when the sole remaining heir of my legacy family keeled over.  (The ghost of his mother spooked him, and he died of fright.  Nice, right?!)  Luckily another family member was on hand in time to plead with the Grim Reaper.

On the up side, pleading with Death isn't as big a deal, since you can now resurrect the dead.  Or try to.  And usually end up bringing them back as a ghost.  And they come back at the age they were when they died, so if they died of old age, you're really not buying them that much time.  But hey, resurrection!  Playable ghost!  That's pretty fun!  (One does get tired of the ghosty sound effects, though.  I get it, you're creepy, can you stop now?)

Of course the biggest difference between Sims 2 and Sims 3 is that the Grim Reaper has become an actual NPC.  After he (and it is a "he") takes your Sim, he will often stick around for a little while to chat or putter about.  In fact, it took persistence (and a shocking amount of murdered Sims), but many players managed to get their relationship high enough with Death to marry him into the family!  A recent patch now prohibits you from having a baby with Grim, but you can still download families with Grim babies in The Exchange.

New deaths will be coming with the World Adventures expansion pack, and it sounds like they will be even more interactive than before.  In The Sims 2, a lot of the deaths were random occurrences - getting stuck in an elevator, or crushed by a Murphy bed, for example.  In World Adventures, you will be able to battle a mummy, for example.  (No word yet on whether you the player actually fight the mummy, or if you just click "Fight Mummy" and let it play out.)