For Screenshots of SC2 follow This Link. There is an awesome Cinematic Trailer that Blizzard has put out you should head over to the SC2 Website and download in the highest resolution that they have available. Below are some gameplay videos with english developer commentary from the South Korean announcement event.
If you look at the screenshots that are out, there is One Screenshot that seems to show Kerrigan the Terran Physic who was abducted by the Zerg and then mutated into a Zerg super unit. So the story will probably follow on from where the BroodWar Expansion left off.
Design wise Blizzard seems to have gone with a "this unit is awesome, but is very weak against this specific thing" design strategy. Which is good because all the best strategy games have very limited numbers of distinct unit types. As long as the base management doesn't turn into micromanagement SC2 is going to be awesome.