Here are some of the creatures that will be wiping out humanity in Mass Effect 2:
Collectors – an insect-like race, controlled by a single leader, that has access to the technology that "no one else in the galaxy" has. They can paralyze humans, and according to the video dev diary above, can abduct entire colonies. Oh, and they have beam weapons.
Abominations – Husks from the first Mass Effect, but now on fire and 100-percent more suicide bomber-y.
Scions – Three transformed human beings that have been grafted together onto a weapon. Essentially, super-zombies. With guns.
Batorians – A "flying suit of armor" apparently made of thirty Husks that can "obliterate everything."
The Geth will also make a return in Mass Effect 2. Legion, a Geth that has gained sentience, will hunt the player through the adventure, and it'll be up to you to figure out why. Provided your Commander Shepard, um, doesn't die first.