When Half Life 1 and 2 came out, there really wasn't anything like them on the market, and gamers ate it up.
If Half Life 3 came out today, using the same formula as 2, it would be a decent game, but like many other shooters already available. Facial movement and speech, physics, presentation and story; these are all now common in games. Half Life's story alone is NOT enough to make a great game; in fact, Half Life's story was great because of the package it was wrapped in.
Valve wants another genre defining installment to get the newest generation of gamers hooked on the franchise, and as a gamer that is what I also want. I don't want more of the same; I want that next game that blows me away, and if it takes 20 years, so be it.