Who cares how old Source is? It still looks fantastic by today's standards IMO... and the more features an Engine has doesn't make it more of a visual treat.. sometimes brilliant art directing and level design overcomes a weaker engine.
With HL2, the only thing really setting it back the most is low-poly objects and low-res textures... FakeFactory's Cinematic mod re-textures everything and replaces most of the character models with high-poly, high-res models that are pretty stunning, here's some screenies:
I don't know - the HL2 just has more atmosphere and better art design in general - I just like it better visually than CoD4 or Crysis. BioShock and Gears is just a disgusting amount of clay/plastic, hardly photo-realistic, although 'graphics' are impressive, just doesn't feel right though, iunno