The levels, models, textures etc. are all the same. So, of course, it looks like a six year old game in that regard.
The lighting has been improved, though in 90% of the situations you will not noticed. Physics has been included, though there are so few physics objects that you may not notice. As I said, the main improvement I've seen thus far is the ragdoll corpses.
Valve have said they ported HL1 to source mainly as an experiment to see what sort of issues mod makers would face when moving their mods to source. It wasn't meant as a project to radically improve the game.
However, now that the first step is done, expect mod makers to do a lot more. I know for a fact that there are some already at work on adding higher res textures, models from Blue Shift and HL2, and other improvements.