According to John Lowrie, a voice actor who has worked on several Valve games including DOTA 2, Left 4 Dead, Half Life 2 and Team Fortress 2, one of the main reasons behind half Life 3 delay is that Valve is waiting for better motion capture technology.
John is not really a technical guy, but he's got his info from developers close to the project. Characters in Half Life 2 look directly at the player when talking to him, but that is not possible when using motion-captured animation. Once a motion is captured using current tech, in-game characters will repeat it exactly without being able to modify it. This works great for movies, but not for interactive media such as games.
"As far as I know they are not developing HL3 now for several reasons, among them the mo-cap issue," the voice actor said in a comment on his personal blog. "Sorry for any confusion. What they might decide in the future depends on lots of different factors. I hope they do, personally, but it has to make sense for them."
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