Semi-open I would say as it has huge levels and freedom to move as it pleases you minus some parts due to storyline.
First level "Island" IMO small/medium compared to other Crysis levels (low-med/high-colorgrading setting btw!
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Which still makes it a very linear on rails game.
You can't do missions out of order. There aren't even any real side missions except for the occasional bonus objective. Additionally you're completely locked to the current zone, which is even more restrictive than say Half-Life 1 where you could travel back to the previous zone (not that I can think of why you'd want to) in some cases.
Crysis, just like Deus Ex, Thief series, original Far Cry, is very much an on rails straight forward linear shooter. It, just like those others, just gives a very nice illusion of freedom and open worldness.
Which is in stark contrast to something like Far Cry 2 where you can go to basically any of the zones whenever you wish. Finish things in whatever order you wish (except at certain points you'll move the "story" along). And where the story (IMO) suffers from the lack of cohesion and forced linearity.
For example there's one mission I remember where one of your buddies is about to be attacked, at any moment now, and you are URGENTLY needed or he'll die. Except there's no real urgency to head there, go immediately or wait a few days. The guys after him will patiently wait until you go help him. Uh...yeah whatever.
Even DAO which I love, I hate the open world aspect. The tweaker in me has me travelling all over to maximize character stats and items. But storywise, it makes no sense when I do that. The blight is about to take over the country but it doesn't matter if you do it in a matter of days or take months (I'm assuming travel on foot takes quite a bit of time). So the story was great, as long as you can ignore the inconsistencies introduced by being able to do things in any order and travel whenever and wherever you wish, taking as long as you like...
Regards,
SB