I know the actual title is HL2: Ep2, but even Gabe has said they consider it HL3.

Anyway, there have been several upgrades for this next epsisode. Dynamic lighting so that models now self shadow. Cinematic physics, so that environments are more destructable. Higher res textures, updated models, new enemies and weapons. Yadda, yadda and the proverbial yadda.

There are several sites with vids. 3DGamers has all of them together for convenience, but Filefront and others have them to. Definately worth checking out.