You can see the full interview here if it grabs your fancy. It's worth a watch, but if you’re like me and you don’t especially care about Half-Life (sorry, I was a bit too young to play), then the thing that will probably make you most curious is his views on the future: namely, we’re apparently only a stone’s throw away from making the Matrix a reality.
"The area that I'm spending a lot of time on has been growing out of a bunch of research that occurred a while ago on brain-computer interfaces… I think that that's kind of long lead stuff, so that's kind of the background thread that I get pulled back into when other things aren't demanding my attention.
We're way closer to The Matrix than people realize. It's not going to be The Matrix—The Matrix is a movie and it misses all the interesting technical subtleties and just how weird the post-brain-computer interface world is going to be. But it's going to have a huge impact on the kinds of experiences we can create for people."
He continues to elaborate how the technology behind human-computer interfaces have advanced dramatically, and how the brain has “really good interfacing for some things”, and that failing to anticipate this future would mean the extinction of any medium who fails to keep up.
So if you’re wondering why there’s no Half-Life 3, it’s probably because Valve’s working on this instead.