I still don't know why people say Half Life had a "riveting plot." Come on. It was Doom regurgitated - Aliens invade, kill, make zombies, guy shoots them all up. The only "plot" twist were the soldiers shooting people instead of saving them. Later add-ons to the game brought some more personal stories of other characters and hints at a metaplot, but HL1 didn't have anything near riveting in the plot range. And I still played it through three times.
The riveting part about HL1 were the AI of the marines, and the innovative levels (such as the tentacle monster in the missile silo). I have seen proof of neither good AI nor innovative levels in either D3 nor HL2. Although, HL2 has the innovative feature of controlling some aliens.
If the Combine soldiers in HL2 are less well done AI-wise than the marines in HL1, despite all other improvements, I'll be heavily disappointed.