I really have a bad gut feeling about HL2, there's just something in my mind that makes me think that Valve was a one-trick pony.
Half-Life came along at the right time, when there was the tech to do some nifty stuff that could really 'wow' an audience, but the people that developed this tech were more concerned in the tech itself rather than the game that went with it, so HL had the perfect stomping grounds to be revolutionary.
But will HL2's entry be as warming? Given, there are still a LOT of things we have yet to touch in an FPS in terms of involvement, but I'm not convinced that we have the tech to do anything mindblowing at this point.
I just get the feeling HL2 will be nothing more than a polished up version of HL with a different storyline.