What interests me is what the designers have DONE with the tools at their disposal. HL2 seems to have a much greater variety in terms of locations, characters (monsters) and gameplay. That's enough to convince me that I will enjoy HL2 more.
It seems to me that Carmack is way out in front of the rest of id, and they're trying to trail behind him. His technology always has potential, but it's far more advanced than the technology it needs to truly use it. Just like one can look at Quake 3 and then look at JK2, and it's astounding that it's the same engine. What the Source engine seems to do is use perfectly the current ability of cards, rather than forcing sacrifices in order to achieve innovation in engine tech. It's reinventing the wheel, rather than attempting to create a prototype of what could replace it.