The Half-Life series is mainly about innovation in singleplayer story-telling tbh. The optimization of the physics engine is impressive, but engines of that scale were starting to get implemented in other games, HL2 was just early. The pretty gwaphics were nice but not crazy ahead of the times, except maybe with the shiny shiny normal maps.
Jeez looking back at the Leaked Half-Life 2 Beta makes me sad on how much was Removed from the Final Product (all those cool and unique Weapons and Enemies), in the Final Version we got a Boring Arsenal and Generic Enemy Roster. Don't get me Wrong, Half-Life 2 is a Good Game and was Revolutionary for the Time/Year/Century it came out, but it gets boring for people like me after playing through it and its Episodes more than twice unlike Half-Life 1 and its Expansion Packs.