Stealth is such a niche with high standards, I did not expect some small indie game to push stealth gameplay so hard and so well.
A good stealth game requires two important components: good AI and good level design. AI is top notch, reactive, smart, aggressive yet careful, it really is the heart of a stealth game, because there's no fun hiding if the enemy won't seek you out. Levels are well designed in that regard, they offer many interesting situations, and options for violence, aggressive stealth, non-lethal stealth, and complete ghosting.
As for extras, it has a nice selection of tools and weapons with quite believable characteristics making it somewhat "realistic", pretty good and highly readable graphics , reactive and well designed controls, extras options for hardcore players, like a realistic magazine system complete with reloading the mags bullet by bullet, and even the music is great.
My major gripe though is doors. It's an issue in plenty games, especially stealth games, even in some AAA titles like Hitman, but here it literally gets in the way: they swing both ways and block corridors way too often, closing them requires being completely out of the way, and they close by magic after AI goes through which make no sense. Also AI can notice you closing a door even though they close on their own, but apparently they can tell the difference, and their alert level to a door is pretty much equal to finding dead body. It often leads to frustration, and this game has TONS of doors which makes this a common occurrence. It takes extra care to avoid getting in these situations, and it's just not very fun, Intravenous 2 could surely use some better door mechanics.