HL:Source was basically jut HL1 running under the Source engine. All maps and everything else exactly the same.
This is like HL1 if it was created now, using today's technologies. Each map is based loosely on the equivalent original HL1 map, but significantly enhanced. I also read somewhere that some of the boring bits have been removed or shortened, and some of the significant parts have been fleshed out.
I have been waiting for this for a few years now. All I have to do is find the time to play it ...