I love the lipsynching and animations. The attention to detail for the characters was fantastic in Half-Life 2. In addition to the immensely fantastic work with the shaders for the levels. The levels are actually much lower in polygon count than Doom 3, but look stunning because of the impressive shader effects. It was impressive seeing an untextured wire mesh of Half-Life 2. It looks rather blocky compared to Doom 3, but they did a fantastic job with the art that you can't even tell at all.
The only reason why I think Halo is the "end-all be-all" of FPS games is because so many of those people have never played Half-Life 1 or 2, or many of the other excellent FPS games out there. Halo had a marketting blitz up the wazoo, and was one of the few solid FPS games on a console that reminded people of GoldenEye (which is another game that was very outclassed in many aspects by PC shooters...that people had never heard of so GoldenEye was the bestest game evah!).
I dunno. The focus on physics, and the gravity gun in particular, were awfully fun. It was cool being able to kill guys up on a bridge by taking out the supports and watching their reactions as they fall to their doom. And grabbing hand grenades and tossing them back with the gravity gun (or even just your hands) was pretty sweet too.