Looking forward to this entirely too much. Oddly enough, one of the things I'm looking forward to are the improved reloading animations. In newest videos I've seen, they look goddamn delicious. One thing that always slightly irritated me in the original. I'm looking at you, MP-5 reloading animation.
And Cremo, one of the biggest new additions is the factions system. There are 5 major factions IIRC, which dynamically fight over resources and terrain on the expanded maps, and you can work for any of 'em. Some of 'em are mutually exclusive naturally, like Duty and Freedom, but I'm pretty sure there are some that'll play nice together.
Also artifacts are rarer and harder to find, so artifact hunting actually should be trickier. Also updated inventory screen and weapons modification system, every weapon being able to get multiple upgrades, and what upgrades you can get depends on who you're allied with and what resources they have available.
Basically, if you liked the creeping around, collecting loot and doing side quests in the original, Clear Sky is supposed to do that a hell of a lot better. Main quest is apparently roughly equivalent to the original.