Yeah, but the first Half-Life ended by explaining the purpose of the guy you'd seen following you throughout the entire game, or at least finally letting him talk. It also answered a bit of what had been going on. Ending Half-Life 2 with the G-Man Ex Machina didn't work nearly as well, because we already know who he is in a basic sense, and he didn't tell us anything new, explain the purpose behind everything that had just happened, or really say anything of worth. He just stepped in at the climactic moment and stopped the game.
