From what I understand, the only difference between the original and getting it via Steam is that there are more bugs. I watched a friend of mine (real good HL player) go through it, getting thwarted by bugs all along the way, the meanest one being that the last elevator in Xen to the boss wouldn't come down (which is how you'd stop someone like Freeman in real life, haha).
If it's HL: Source, the only difference is that some of the physics come into play (again, more bugs than by design from what I understand) and that they make use of 3D skyboxes and water effects. They don't really update the look of the levels, models, or textures though. This is why I'm anxiously awaiting the release of the Black Mesa mod.
The Plothole: a home for amateur, inclusive, collaborative stories
http://forums.theplothole.net