One of the things which really surprised me when I played HL and all its subsequent versions(HL2, HL2:E1, Hl2:E2) was the FPS game design -- The designers knew precisely the limits they could go to make it damn difficult for you to find next steps you will need to do and what to do to reach to those steps. Combine that with FPS gameplay and you have a heck of a game there. Loved everybit of my life playing it and that's why I choose HN username as "gordon_freeman". :)