Honestly, I didn't really reflect all that much - if at all really - on that aspect until this thead cropped up. I just felt the HL universe took a gigantic left turn somewhere inbetween the first and the second games, and it felt like the two installments were totally disconnected. Other than the crowbar and HUD graphics, what really says Half-Life about Half-Life 2? Almost nothing.
It doesn't exactly help either that you as a player basically get no information whatsoever of what happened during the intervening twenty or so years that passed between the first two half-lives. That's the bit that really bugs me. Gordon isn't allowed to talk, so he can't simply ASK what the EFF happened to the world, because that's the (stupid) design decision made long ago, and nobody in the gameworld sees that as strange. The NPCs even joke about it, which rather breaks the suspension of disbelief IMO. We also get no real explanation of how exactly the director of Black Mesa ends up as supreme (and rather deranged) leader of all of mankind. Or rather the small splinter of it that seems to remain... As amusing as the rants of Dr. Breen was, he's as retconned into the story as Magnusson was in Ep. 2. And to top off the cake, add the lack of geographical connection with any real-world locations. City 17 (which is a terrible name by the way) is supposedly set in Russia, but it has plenty of english signs everywhere. Also, nobody actually speaks any russian, or even english with a russian accent... Meh!
It's not the same game anymore; the style of gameplay, the atmosphere, everything... It's all changed. The original HL wasn't perfect, god knows it wasn't. For starters, Black Mesa was just way, WAY too big to be realistic, and not to mention too random and computer-level:y. There isn't even any bathrooms in the entire game that I can recall other than the locker room right at the start... Then add all the environmental hazards the player has to negotiate. Bizarre trips on conveyor belts, random crushers that exist for no reason (other than act as a lethal obstacle for the player of course), vast pools of toxic waste, gigantic pits that serve no apparent function and so on.
Even with all that in mind, it's still on the whole a better game in my opinion, because it had a particular mood.
I agree with what you said.
But I will defend HL. The level design was perfect for its time. Before Gordon gets the suit, he's already in the bathroom. I don't think it would have added MORE by having bathrooms. It would just be redundant. For a game of that time, it was HIGHLY interactive. Some parts required more than just a) shoot enemy b) get key to open door. It was interesting, fresh.
The story of HL2 bothered me as well. In HL1, we are led to be believe that Black Mesa is a real underground lab in the the real world, which to me, seems more believable than Valve's futile attempt at bringing the ENTIRE WORLD into the Black Mesa incident. They haven't told the entire story of how the world came to be in HL2, not because for a more revealing story telling, but rather the scope is set too LARGE to be explainable. How can the entire planet revolve around Gordon Freeman and the few side characters? I too have a hard time connecting the two games together.
Lastly the NPCs of the first game fitted the rising situation where destruction and chaos lurked every corner. In HL2, as great as the NPCs may look, they are just dull to listen to. It doesn't fit when everyone on the entire planet loves Gordon Freeman, such a person would not exist if they were trying to properly blend realism with fantasy. It just doesn't work.
HL2 left too many questions, it could not be properly answered on its own. It feels more like a setup for a trilogy (which of course it is) than a solid story of its own. HL1 on the other hand, could have been easily left at where it concluded and it still would satisfy the requirement of a perfect sci-fi story.