Anything and everything you could think of. Now before you start thinking about how none of these really have to do with the games companies makes, take a look at a few that have lived, and died. Or barely even existed. Now keep in mind, this isn't just Half-Life based, though I cover that later. Products thrive on hype.
Games are the same way.
You have to hype it, to sell it. Pump up the little puppets they know as customers to ecstatic levels to buy these games. We'll start with Quake. Fairly hyped, but it had reason to be. It lived up to expectations.