Dead Space 2: Severed is a shorter, harder, and more action oriented Dead Space 2.
Storyline:
Storywise it's a sequel to Dead Space: Extraction. It follows the storyline of Gabe Weller and Lexine Murdoch (now Lexine Weller) who have been on The Sprawl for the past 3 years. It shows their story on The Sprawl, and how they fought for survival. Like all Dead Space media this has an excellent story, however it has a near tear dropping ending if you've played Dead Space: Extraction.
Game-Play:
It's more Dead Space 2, so of course it's excellent.
Graphics:
It's more Dead Space 2, again quite excellent.
Sound:
For the most part it's Dead Space 2's sound design, albeit new voice actors as most of the cast from Dead Space 2 isn't present here (though you hear Tiedeman's voice). The voice acting is FANTASTIC. The actors for Gabe and Lexine reprise their roles and they do a lovely job.
Overall:
If you loved Dead Space 2, you'll love Severed. It's totally worth the $7 if you ask me. Though the story isn't quite as interesting as Dead Space 2's simply because Gabe isn't quite as interesting of a character as Isaac is, it's still very enjoyable.