It is a pretty open world. Can't compare it to HR as I've only played the original. But, while you start out on Liberty Island, you'll get to explore more of New York and several other places around the world. I won't list them all out, it's better to not know everything ahead of time.
Stealth can be pretty helpful. I'm not one of those crazy people who plays the entire game without shooting anyone, but it can make a considerable difference. And the aiming system does take a lot of getting used to.
Also, the first mission is not particularly easy. There are a lot of chances to get tripped up. Save often, you never know when there's a tripbomb around the corner. And Kozmos is right... it took me 35-40 hours the first playthrough, but after that, when I had an idea where hazards were, it went a fair amount quicker.