In the mod, the combat and puzzles will be like nothing you've ever seen in a Half-Life game. Puzzles will require extensive use of the gravity gun and combat will leave you speechless. Here are some quotes from playtesters of one of the dev maps:
"The combat was extraordinary!"
"It was really, really fun."
You will be able to kill enemies in a whole new way and experience entirely new sections of the Half-Life universe. You will explore the depths of the arctic, the heights of the Combine architecture, and see a renewed version of Xen, Combine infested and all.
Make sure to check out The Orange Suit Project when it comes out... which may take a while.