Yea, there were some weird games on this list. I'm sure they didn't really talk about Half-Life one because Half-Life 2 was on the list, but I think the original Half-Life was a defining FPS game that ought to be one of the best FPS games out there. I also have no idea how Doom 3 gets on this list....
Whatever, I'm glad that rather than go into the hype of the Halo series, the first Halo is the one that makes it onto this list.