KnightBrolaire
SSO's unofficial pickup tester
I thought this was a very interesting (though a bit too technical and verbose) look at why the original crysis still gives modern PCs a lot of trouble. the TLDR version is basically the way it utilizes processing power and the fact that Intel assumed at the time that we'd all be using super high clockspeed quad core processors (instead of our current multi-threaded processors). There's other variables like the way it renders lighting and effects real time, and the physics/general interactivity with objects in the world.
I think the game holds up incredibly well to this day, and the fact that you could interact with your environment/items in the environment so freely is something that many games are lacking. Hell, that concept of extreme interactivity and systems driven gameplay is part of why I loved Prey and am currently loving Underworld: Ascendant.