In HL1 the player is introduced to the Headcrab’s coupling behaviour after seeing a headcrab sitting atop a scientists head.
In HL2, this experience is presented in a more cinematic event; the player watches helplessly as a citizen struggles for his life after being mounted by a headcrab.
With Source’s cinematic capabilities, I would like to see a brutal and dramatic zombification as the player’s introduction to headcrabs and zombies. Or at least tell me you guys have something cooler in store besides a headcrab humping a dead scientist.