I'm just the opposite. To me, HL1 was run of the mill. It was nothing special. Half Life 2, on the other hand, is cooler than the other side of the pillow in my mind. I don't think the game was especially long, but I think it was long enough to feel satisfied, and more importantly, it felt epic. It felt real. You were put in place by the G-man. To do what, you knew not, but in the end, you stopped the combine, and even if that meant the sacrifice of everyone who helped you along the way, it was for the greater good in the end.
Whether you knew it or not, that was why you were there. G-man knew it. Everyone else knew it. You didn't, but you succeeded, because for some reason, Gordon has a tendency to do that. Which, after all, is why you were...recruited by the G-man in the first place. The right man in the wrong place can make all the difference in the world. It is what Gordon does, be that right man, and I have no doubt that will continue to be the trend in his future tasks.
I would also like to comment on the lack of boss battles and give that my praise. Nothing pisses me off in first person shooters like stupid bosses that do nothing but make me familiarize myself with the loading screen. Half Life 2 eschews boss battles in face of challenging set pieces, like the two gunships you must face to enter Nova Prospekt, or the three striders you must take down in the courtyard. It is for the better. These conflicts feel more real than boss battles - in real life, there are conflicts, but there are no boss battles. It is the same in Half Life 2 - you are part of a struggle, not a mano a mano duelist. I hope other developers see how well this works and follow this trend. The day boss battles die will be a good day.
I think the ending worked because of this. You had already won, battering your way to heart of the combine citadel. It was simply a rubber stamp to destroy the reactor and Dr. Breen and secure the victory for the same people you fought for. It was the end of the story - what needed to happen after your long, hard slog. It concluded the events that took place in City 17 in the near future, and now, Gordon must move on to his next "assignment".
I have to say, as bittersweet as it is, I really enjoy Half Life's method of storytelling. There is an extent to which the observer should be left wondering, because revealing always ends the wonderment and sense of awe. Stephen King once said: "If the door opens and there's a 10 foot bug standing there, part of you still goes 'whew! at least it's not a 20 foot bug!'"
He has a point. I think the mysteries of what happened between Black Mesa and City 17, who the combine are and what they do, are stories that may or may not be for another time. Regardless, I think we get better results from the conjecture in our minds than if Valve corrected our whimsy and removed elements that made the enemy so vile to us.
Wow. That post ended up being a lot longer than I thought it would be.
Regardless, this is my feeling towards the narratives of the Half Life games. Think of it what you will.