"Every HL map can be equally made with the Source engine, but not every Source engine map can be done with HL. "For example, I started some sort of remake of my map Tinnos in Source. I had to make textures bigger and added bumpmaps, water etc. The Source map looks better, no doubt. It needs an extra effort, but it looks extra nice.
Another one of my maps: Kaufmann House. Two years ago, the possibility of making that map (and looking anything like the real place) was just a wild dream. Does anyone want to remake that for HL1...? I thought so.
And finally, let's not forget about:
-Displacement surfaces! Are you sick of working with triangles that slow down the VIS process? This is for you!!
-Lightmap grid: Do you want shadows to look good without having to lower the texture resolution? Here's your chance!!
-Func_details: How many portal leafs does your map really need?
-Shaders: Water finally looks like... water!!!
The list goes on and on