I also finished HL2 for the first time about two weeks ago.
The bulk of the game was pretty fun, and the environments really, really good. The pace was jarring, though. I wonder whether it was a mistake to play on Hard. Too many times I felt it simply wasn't possible to get through without abusing the AI a ton (and knowing where they will spawn), and that's hardly a recipe for immersion. Based on this experience, when I eventually play Ep 1, I will do so on Normal and see how that goes. Hard difficulty should never thicken the opponents' armor or give them perfect aim, but spawn more of them into better positions, make their team AI better and more aggressive, things like that.
The most annoying and/or lackluster parts I remember were the homing missiles on Route Kanal, the rollermines on Highway 17, and the Citadel after the fighting started.
4/5 stars.