Imagine the original Deus Ex as a sci-fi survival horror game, with a more complex skill/inventory system. I'm not saying it's perfect, and it hasn't aged without a few wrinkles, but the sheer scope of it is amazing. There are tons of different mechanics to exploit or ignore - including combat, stealth, hacking, research, dozens of "magical" skills - and the game really lets you tackle its challenges in whatever way suits you; provided you use your resources right and stay concentrated.
Bah, I'm bad at explaining things. Just turns into blather. Survival Horror/Sci-Fi Deus Ex fits it well enough, assuming you've played Deus Ex.