Blizzard has unveiled the new Death Animations for various Zerg units.  They simply look AWESOME.  Very realistic. Probably depend on what unit has killed them.  You can see some Terran units burning Hydralisks and Lurkers.  They are literally set ablaze, and slowly get consumed by fire, until their corpses desintegrate.  When Marines kill Zerg units, their body parts get dismembered, and you can see body parts shaking and moving before staying completely still.  The death animations do add a very special touch of realism in contrast with the original RTS game where a splash of blood served as death animation.

Blizzard Quote:
etween adding new unit models and sliming up the zerg buildings, the StarCraft II art team has also spent some time on some little details that make the game come alive, such as new unit death animations. Featured below, we have a protoss carrier being blown out of the sky by a squad of terran marines, as well as several zerg units falling to their fates.