I'm currently playing through the second HL and this statement strikes me as nonsense.
You're entitled to your opinion good sir, just like me. So am I allowed to nonsense your nonsense strike? - I jest.
HL1 was emotional and memorable, with some predictable parts and wonky jumping sections.
HL2 was stale with a predictable story line and felt, again to me, more like a physics tech demo than the second HL.
(Seriously, how many times was physics the answer to a puzzle in that game - Can it be shot, no.. Is there water... Oh yeah.. stupid player.)
Maybe that's down to the amount of games that used the same type of strategies Valve did. EG:
The lift will be here in 5 mins = Defend the lift. (IIRC it's from the Ep content rather than HL2 but meh, it's for example)
We don't go to Ravenholm anymore = I'm going there then, it will also be dark and scary because they said they don't go in a sad voice)
Or even just I got older and my world view and expectations changed, but HL2 didn't move me like HL1 did. HL1 is still my standard to beat in a game. (that and System Shock 2, and I hold little hope for SS3 to even come close tbh)
Also, the creative writing behind HL1 have all left Valve IIRC leaving a bunch of people who have done thing but make hats for years and eat free buffet food.
Not a talent Id look for in creative writers personally but they might pull something out their used (tea)bag, we never know. So here's too the undying hope of fans of the series, to which I am overall.