I wouldn't say the gameplay of HL2 was groundbreaking except the gravity gun, that was a blast. The AI was though. There are still very few gaming characters that can match up to Alyx Vance. But the HL games were just really fun and well made. The animations (going down the mineshaft and seeing the spiders crawling around below you, or the semi-organic warships for instance), the variety in level types, the scale of the world, the dialogue... many of those things are still lacking in modern FPS games.
I actually wanted to come and post something similar.
HL made a marked impact on the way FPS games told their story, by changing the way exposition happened and keeping the player in control at all times. HL's level design made you look in the places Valve wanted you to look, without ever taking control from you. It was a truly massive leap forward in many regards. Exposition via gameplay was HL's lasting effect on me. HL2, on the other hand, didn't have any such technical improvement. It definitely built on HL, doing everything right that HL did, and improved the GFX and AI, but not to a point where I was ever awestruck. It was a good shooter, but in the long run I actually believe that COD4:MW had a larger impact on the genre and it's progression.