HL1: I was saving my ass (saving the earth was a simply a consequence)
HL2: I was being forced to save the earth without a choice in the matter.
A clear example about how HL2 would be more like HL1: I would be able to kill alyx and take her gun & ammo (same for any other friendly characters), thus scripting wouldn't rely on her being alive or anybody else (everything would just happen around you)
Agree with what you said about the scripting...
Also there is the issue of us becoming a lot more jaded. For example back in 1998 simply the fact that there were so many friendly NPCs around me was impressive. The fact that I could get them to follow me around, and that they would make random comments and heal me or fight by my side- all that was a big deal at the time and made the game feel real.
Fast forward to 2004 and despite trying very hard the character interaction in HL2 does not feel organic compared to certain other games out at the time- specifically role playing games like Knights of the Old Republic. Maybe this is not any fault on Valve's part- perhaps it's just a genre limitation. But the bottom line is now that we have gotten used to more intimate character interaction via RPGs like Mass Effect it's not easy to be impressed with a script like Kliener's lab in HL2. It feels more like a watching a movie than playing a game where you are the protagonist. I think it could still work if they were able to create scripts that were truly compelling to sit through thanks to story revelations- but the in game story is not too gripping and the big questions are never tackled so that doesn't work either.