The combat may not have been the most interesting versus basic grunts, but it never got stale. I've never played another game where the core gameplay changed so much so frequently.

Physics interactions -> Basic FPS -> Fan Boat -> Mounted Gun -> Gravity Gun -> Zombies & Traps -> Car -> THE CRANE FIGHT -> Rockets & Gunships -> Ant Lions -> Ant Lion Minions -> Turrets -> Resistance Squads -> Striders -> Super Gravity Gun

Honestly the HL1 combat may have been somewhat more challengjng, but it was a grind. Fights were often just frustrating. I've abandonded playthroughs because I didn't feel like spending another 10 hours beating my head against the endless amounts of enemies just to get to the end of... whatever I was doing I forgot.

HL1's big innovation was never removing control from the player just to tell the story. Beyond that they also had some interesting AI behaviour and weapons. It was a game with old-school length and old-school difficulty.

HL2's big innovation was the physics engine, and they played with it in so many ways, whole polishing every other aspect of the design. They kept the gameplay tight and did something just long enough to explore it and then they moved on. They never forced you to hang out just repeating the same loop over and over to pad the length.