Ubisoft clearly says that it's not really a remake, but rather a reimagining of the original game. Paul Cuisset says also that they aim to do things that couldn't be done in the original for technical reasons, and also to do something actual, not a old game look newer.
It's still about Conrad and Morphs, but while saving Earth, he has to save his girlfriend too.
It includes new items, new gears, a new experience system, expanded story with a few new twists, and some more. And everything in Unreal Engine 3 and Havok.
There's also artworks:
Sources:
http://www.gamekult.com/actu/zoom-flashback-hd-A108358.html (French)http://www.neogaf.com/forum/showthread.php?t=538329
(Sorry for the french sources, but I'm french and that's pretty much how I knew about the game)