What was your first reaction to… (in HL2)
Lamarr?
Vortigaunts?
(In HL1)
All of the aliens?
(In portal)
The turrets?
GLaDOS?
The lack of aliens?
Personally, my first time with HL2 was with a friend who figured out how to cheat before they updated it, so I walked around with a magnum everywhere, so when Lamarr jumped at me, I shot her accedentally and Kleiner kept talking to her like nothing happened.
The same with the vortigaunt in the train thing, I jumped in with my shotgun, paniced and shot it. Since I played some of Blue Shift before HL2, I hated those bastards.