Oh, the physics are FAR more advanced than Doom 3, but it has been admitted by Newell that the Doom 3 engine produces higher detail levels... I'll try and find the article, but I think it's because Doom 3 is mostly enclosed, while the Source engine has much greater draw distances to figure in.
But, overall, Half Life 2 kicks the shit out of Doom. On so many levels. Sure, Doom gives you that palpable apprehension, almost fear, but Half Life 2 feels <i>real</i>. Strange, saying that about a computer game. But the way the NPC's walk around, they look genuinely frightened. Oppressed. It's marvellous.
And what about the magnifying glasses, eh? What a touch.
-Mike
PS: As for problems, the only gripe I've had is that it took effing AGES to register with Steam. But then again, it WAS the day of release... the game did crash once, but that was due to me being a complete fucktard and trying to Alt-Tab...
