Excuse you, the antlions were amazing. There is one point where there are enemies on a catwalk high above you and if you throw the bugbait at them the Antlions will leap 100 feet up up a sheer cliff to maul them. Glorious.
The first game holds up well because it relied on core ideas for even modern FPSs. Which is the immersive first person perspective, a high density of interesting objects many of which can be interacted with, and run and gun shooting (which in my opinion is the only kind of shooting that gives good a SP experience). These things can still make a shooter interesting to this day (see: the first Bioshock).
HL 2 still has the density of interesting stuff, but in a different way: each section of the game relies on something different. This doesn't work as well because a lot of the sections of the game are based on uninteresting stuff like car driving or long ranged Combine firefights. A lot of the new stuff is also gimmicky; the human allies are fine but you can't command them well and they're only there in environments they have difficulty navigating, the physics puzzles seem sort of pathetic now that physics is casually integrated into every game even those that don't need it. The main problem is that with the new environments are so large that the Xen enemies are not threatening at all so too much of the game is firefights where both sides have hitscan weapons. And you can also go far too long without seeing anything of interest *cough vehicle scenes cough*.
BUT, because so much of HL2 is giving each area a new mechanic to see what sticks, there are some really great sections. The turret sections in the prison. The points where your resistance allies are actually working with you instead of getting stuck on the environment. The guided missile dance to hit gunships. The joy of sicing alien monsters on your enemies for once. The fact that Vortigaunts and a giant pet robot are your friends now. The super-powered rampage. I also like the art style, it tells you much more about the world than the actual dialogue does.
It also has the immeasurable advantage of, you know, not having an entire quarter of the game be unworthy of the series' name.